diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec85a16072e..65314a89091 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 - Updated `tokenKeyword` to match the definition of keyword field type ([#5251](https://github.com/elastic/eui/pull/5251))
 - Added `element`, `buttonElement`, and `arrowProps` props to further customize `EuiAccordion` ([#5258](https://github.com/elastic/eui/pull/5258))
+- Fixed missing `id` for `EuiCombobox` by generating one if `prepend` or `append` exists ([#5229](https://github.com/elastic/eui/pull/5229))
 
 **Breaking changes**
 
diff --git a/src-docs/src/views/combo_box/combo_box_example.js b/src-docs/src/views/combo_box/combo_box_example.js
index bfafa4b18c1..44fc6237d7e 100644
--- a/src-docs/src/views/combo_box/combo_box_example.js
+++ b/src-docs/src/views/combo_box/combo_box_example.js
@@ -75,8 +75,10 @@ const groupsSnippet = `<EuiComboBox
 />`;
 
 import SingleSelection from './single_selection';
+import SingleSelectionPrepend from './single_selection_prepend';
 const singleSelectionSource = require('!!raw-loader!./single_selection');
 const singleSelectionHtml = renderToHtml(SingleSelection);
+const singleSelectionPrependSource = require('!!raw-loader!./single_selection_prepend');
 const singleSelectionSnippet = `<EuiComboBox
   placeholder="Select a single option"
   singleSelection={{ asPlainText: true }}
@@ -84,6 +86,13 @@ const singleSelectionSnippet = `<EuiComboBox
   selectedOptions={selectedOptions}
   onChange={onChange}
 />`;
+const singleSelectionPrependSnippet = `<EuiComboBox
+  prepend="Prepend"
+  singleSelection={{ asPlainText: true }}
+  options={options}
+  selectedOptions={selectedOptions}
+  onChange={onChange}
+/>`;
 
 import SingleSelectionCustomOptions from './single_selection_custom_options';
 const singleSelectionCustomOptionsSource = require('!!raw-loader!./single_selection_custom_options');
@@ -402,19 +411,39 @@ export const ComboBoxExample = {
               {'singleSelection={{ asPlainText: true }}'}
             </EuiCode>
           </p>
+        </Fragment>
+      ),
+      props: { EuiComboBox, EuiComboBoxOptionOption },
+      snippet: singleSelectionSnippet,
+      demo: <SingleSelection />,
+    },
+    {
+      title: 'Single selection with prepended label',
+      source: [
+        {
+          type: GuideSectionTypes.JS,
+          code: singleSelectionPrependSource,
+        },
+        {
+          type: GuideSectionTypes.HTML,
+          code: singleSelectionHtml,
+        },
+      ],
+      text: (
+        <Fragment>
           <p>
-            <strong>Note:</strong> <EuiCode>append</EuiCode> and{' '}
-            <EuiCode>prepend</EuiCode> props only work if
+            <EuiCode>append</EuiCode> and <EuiCode>prepend</EuiCode> props only
+            work if
             <EuiCode>singleSelection</EuiCode> prop is not set to{' '}
-            <EuiCode>false</EuiCode> to avoid multilines that makes combobox
+            <EuiCode>false</EuiCode> to avoid multi-lines that makes combobox
             height greater than that of <EuiCode>append</EuiCode> and{' '}
             <EuiCode>prepend</EuiCode>.
           </p>
         </Fragment>
       ),
       props: { EuiComboBox, EuiComboBoxOptionOption },
-      snippet: singleSelectionSnippet,
-      demo: <SingleSelection />,
+      snippet: singleSelectionPrependSnippet,
+      demo: <SingleSelectionPrepend />,
     },
     {
       title: 'Single selection with custom options',
diff --git a/src-docs/src/views/combo_box/single_selection.js b/src-docs/src/views/combo_box/single_selection.js
index 9293e94e3f6..e8212a6bb76 100644
--- a/src-docs/src/views/combo_box/single_selection.js
+++ b/src-docs/src/views/combo_box/single_selection.js
@@ -51,9 +51,9 @@ export default () => {
       canDisabled={false}
       canReadOnly={false}
       canLoading={false}
-      canPrepend={false}
       canIsDisabled
       canAppend
+      canPrepend
     >
       <EuiComboBox
         placeholder="Select a single option"
diff --git a/src-docs/src/views/combo_box/single_selection_prepend.js b/src-docs/src/views/combo_box/single_selection_prepend.js
new file mode 100644
index 00000000000..4682b21e78b
--- /dev/null
+++ b/src-docs/src/views/combo_box/single_selection_prepend.js
@@ -0,0 +1,57 @@
+import React, { useState } from 'react';
+
+import { EuiComboBox } from '../../../../src/components';
+
+const options = [
+  {
+    label: 'Titan',
+    'data-test-subj': 'titanOption',
+  },
+  {
+    label: 'Enceladus',
+  },
+  {
+    label: 'Mimas',
+  },
+  {
+    label: 'Dione',
+  },
+  {
+    label: 'Iapetus',
+  },
+  {
+    label: 'Phoebe',
+  },
+  {
+    label: 'Rhea',
+  },
+  {
+    label:
+      "Pandora is one of Saturn's moons, named for a Titaness of Greek mythology",
+  },
+  {
+    label: 'Tethys',
+  },
+  {
+    label: 'Hyperion',
+  },
+];
+
+export default () => {
+  const [selectedOptions, setSelected] = useState([options[2]]);
+
+  const onChange = (selectedOptions) => {
+    // We should only get back either 0 or 1 options.
+    setSelected(selectedOptions);
+  };
+
+  return (
+    <EuiComboBox
+      prepend="Prepend"
+      singleSelection={{ asPlainText: true }}
+      options={options}
+      selectedOptions={selectedOptions}
+      onChange={onChange}
+    />
+  );
+};
diff --git a/src/components/combo_box/__snapshots__/combo_box.test.tsx.snap b/src/components/combo_box/__snapshots__/combo_box.test.tsx.snap
index 87ebdc89ef7..a31cea0afc6 100644
--- a/src/components/combo_box/__snapshots__/combo_box.test.tsx.snap
+++ b/src/components/combo_box/__snapshots__/combo_box.test.tsx.snap
@@ -27,6 +27,7 @@ exports[`EuiComboBox is rendered 1`] = `
           <input
             aria-controls=""
             data-test-subj="comboBoxSearchInput"
+            id="generated-id__eui-combobox-id"
             role="textbox"
             style="box-sizing:content-box;width:1px"
             value=""
@@ -70,6 +71,7 @@ exports[`props autoFocus is rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -100,6 +102,50 @@ exports[`props autoFocus is rendered 1`] = `
 </div>
 `;
 
+exports[`props custom ID is rendered 1`] = `
+<div
+  aria-expanded={false}
+  aria-haspopup="listbox"
+  className="euiComboBox"
+  onKeyDown={[Function]}
+  role="combobox"
+>
+  <EuiComboBoxInput
+    autoSizeInputRef={[Function]}
+    compressed={false}
+    fullWidth={false}
+    hasSelectedOptions={true}
+    id="test-id-1"
+    inputRef={[Function]}
+    isListOpen={false}
+    noIcon={false}
+    onChange={[Function]}
+    onClear={[Function]}
+    onClick={[Function]}
+    onCloseListClick={[Function]}
+    onFocus={[Function]}
+    onOpenListClick={[Function]}
+    onRemoveOption={[Function]}
+    rootId={[Function]}
+    searchValue=""
+    selectedOptions={
+      Array [
+        Object {
+          "label": "Mimas",
+        },
+        Object {
+          "label": "Iapetus",
+        },
+      ]
+    }
+    singleSelection={false}
+    toggleButtonRef={[Function]}
+    updatePosition={[Function]}
+    value="Mimas, Iapetus"
+  />
+</div>
+`;
+
 exports[`props delimiter is rendered 1`] = `
 <div
   aria-expanded={false}
@@ -113,6 +159,7 @@ exports[`props delimiter is rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -156,6 +203,7 @@ exports[`props full width is rendered 1`] = `
     compressed={false}
     fullWidth={true}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -196,6 +244,7 @@ exports[`props isClearable=false disallows user from clearing input when no opti
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={false}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -229,6 +278,7 @@ exports[`props isClearable=false disallows user from clearing input when options
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -271,6 +321,7 @@ exports[`props isDisabled is rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isDisabled={true}
     isListOpen={false}
@@ -324,6 +375,7 @@ exports[`props options list is rendered 1`] = `
           <input
             aria-controls="generated-id_listbox"
             data-test-subj="comboBoxSearchInput"
+            id="generated-id__eui-combobox-id"
             role="textbox"
             style="box-sizing: content-box; width: 2px;"
             value=""
@@ -632,6 +684,7 @@ exports[`props selectedOptions are rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -675,6 +728,7 @@ exports[`props singleSelection is rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={false}
     noIcon={false}
@@ -706,7 +760,7 @@ exports[`props singleSelection prepend and append is rendered 1`] = `
 <div
   aria-expanded={true}
   aria-haspopup="listbox"
-  className="euiComboBox euiComboBox-isOpen"
+  className="euiComboBox euiComboBox--prepended euiComboBox--appended euiComboBox-isOpen"
   onKeyDown={[Function]}
   role="combobox"
 >
@@ -716,6 +770,7 @@ exports[`props singleSelection prepend and append is rendered 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={false}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={true}
     noIcon={false}
@@ -845,6 +900,7 @@ exports[`props singleSelection selects existing option when opened 1`] = `
     compressed={false}
     fullWidth={false}
     hasSelectedOptions={true}
+    id="generated-id__eui-combobox-id"
     inputRef={[Function]}
     isListOpen={true}
     noIcon={false}
diff --git a/src/components/combo_box/combo_box.test.tsx b/src/components/combo_box/combo_box.test.tsx
index cafdf0faaa1..5e1be4e4815 100644
--- a/src/components/combo_box/combo_box.test.tsx
+++ b/src/components/combo_box/combo_box.test.tsx
@@ -92,6 +92,18 @@ describe('props', () => {
     expect(component).toMatchSnapshot();
   });
 
+  test('custom ID is rendered', () => {
+    const component = shallow(
+      <EuiComboBox
+        id="test-id-1"
+        options={options}
+        selectedOptions={[options[2], options[4]]}
+      />
+    );
+
+    expect(component).toMatchSnapshot();
+  });
+
   describe('isClearable=false disallows user from clearing input', () => {
     test('when no options are selected', () => {
       const component = shallow(
diff --git a/src/components/combo_box/combo_box.tsx b/src/components/combo_box/combo_box.tsx
index fb460d58e9a..e493eedc20c 100644
--- a/src/components/combo_box/combo_box.tsx
+++ b/src/components/combo_box/combo_box.tsx
@@ -936,6 +936,9 @@ export class EuiComboBox<T> extends Component<
       matchingOptions,
     } = this.state;
 
+    // Make sure we have a valid ID if users don't pass one as a prop
+    const inputId = id ?? this.rootId('_eui-combobox-id');
+
     // Visually indicate the combobox is in an invalid state if it has lost focus but there is text entered in the input.
     // When custom options are disabled and the user leaves the combo box after entering text that does not match any
     // options, this tells the user that they've entered invalid input.
@@ -946,6 +949,8 @@ export class EuiComboBox<T> extends Component<
     const classes = classNames('euiComboBox', className, {
       'euiComboBox--compressed': compressed,
       'euiComboBox--fullWidth': fullWidth,
+      'euiComboBox--prepended': prepend,
+      'euiComboBox--appended': append,
       'euiComboBox-isDisabled': isDisabled,
       'euiComboBox-isInvalid': markAsInvalid,
       'euiComboBox-isOpen': isListOpen,
@@ -1030,7 +1035,7 @@ export class EuiComboBox<T> extends Component<
           }
           fullWidth={fullWidth}
           hasSelectedOptions={selectedOptions.length > 0}
-          id={id}
+          id={inputId}
           inputRef={this.searchInputRefCallback}
           isDisabled={isDisabled}
           isListOpen={isListOpen}
diff --git a/src/components/combo_box/combo_box_input/combo_box_input.tsx b/src/components/combo_box/combo_box_input/combo_box_input.tsx
index 08fb4c57e3d..8200f0adb3d 100644
--- a/src/components/combo_box/combo_box_input/combo_box_input.tsx
+++ b/src/components/combo_box/combo_box_input/combo_box_input.tsx
@@ -264,6 +264,7 @@ export class EuiComboBoxInput<T> extends Component<
       <EuiFormControlLayout
         icon={icon}
         {...clickProps}
+        inputId={id}
         isLoading={isLoading}
         compressed={compressed}
         fullWidth={fullWidth}
diff --git a/src/themes/eui-amsterdam/overrides/_combo_box.scss b/src/themes/eui-amsterdam/overrides/_combo_box.scss
new file mode 100644
index 00000000000..b536b791c11
--- /dev/null
+++ b/src/themes/eui-amsterdam/overrides/_combo_box.scss
@@ -0,0 +1,47 @@
+.euiComboBox {
+  // Round the left border when we append a label
+  &--appended {
+    .euiFormControlLayout__childrenWrapper {
+      border-top-left-radius: $euiFormControlBorderRadius;
+      border-bottom-left-radius: $euiFormControlBorderRadius;
+    }
+
+    // Half size rounded left corners when compressed
+    .euiFormControlLayout--compressed {
+      .euiFormControlLayout__childrenWrapper {
+        border-top-left-radius: $euiFormControlCompressedBorderRadius;
+        border-bottom-left-radius: $euiFormControlCompressedBorderRadius;
+      }
+    }
+
+    // No rounded corners when we have both appended and prepended labels.
+    // This includes full height and compressed height comboboxes.
+    &.euiComboBox--prepended {
+      .euiFormControlLayout__childrenWrapper {
+        border-radius: 0;
+      }
+
+      .euiFormControlLayout--compressed {
+        .euiFormControlLayout__childrenWrapper {
+          border-radius: 0;
+        }
+      }
+    }
+  }
+
+  // Round the right border when we prepend a label
+  &--prepended {
+    .euiFormControlLayout__childrenWrapper {
+      border-top-right-radius: $euiFormControlBorderRadius;
+      border-bottom-right-radius: $euiFormControlBorderRadius;
+    }
+
+    // Half size rounded right corners when compressed
+    .euiFormControlLayout--compressed {
+      .euiFormControlLayout__childrenWrapper {
+        border-top-right-radius: $euiFormControlCompressedBorderRadius;
+        border-bottom-right-radius: $euiFormControlCompressedBorderRadius;
+      }
+    }
+  }
+}
diff --git a/src/themes/eui-amsterdam/overrides/_index.scss b/src/themes/eui-amsterdam/overrides/_index.scss
index 43f603d65f8..ef9f0660d74 100644
--- a/src/themes/eui-amsterdam/overrides/_index.scss
+++ b/src/themes/eui-amsterdam/overrides/_index.scss
@@ -7,6 +7,7 @@
 @import 'card';
 @import 'code_block';
 @import 'color_stops';
+@import 'combo_box';
 @import 'comment';
 @import 'date_picker';
 @import 'date_popover_button';