diff --git a/packages/webpack-cli/__tests__/ZeroConfigGroup.test.js b/packages/webpack-cli/__tests__/ZeroConfigGroup.test.js
index 4b25801772d..7e34eb7dac3 100644
--- a/packages/webpack-cli/__tests__/ZeroConfigGroup.test.js
+++ b/packages/webpack-cli/__tests__/ZeroConfigGroup.test.js
@@ -86,4 +86,12 @@ describe('ZeroConfigGroup', function () {
         // ensure no other properties are added
         expect(result.options).toMatchObject({ mode: 'development' });
     });
+
+    it('should set mode=production by default', () => {
+        const group = new ZeroConfigGroup([{}]);
+
+        const result = group.run();
+        // ensure no other properties are added
+        expect(result.options).toMatchObject({ mode: 'production' });
+    });
 });
diff --git a/packages/webpack-cli/lib/utils/cli-flags.js b/packages/webpack-cli/lib/utils/cli-flags.js
index 9a32f5544af..b2076f95959 100644
--- a/packages/webpack-cli/lib/utils/cli-flags.js
+++ b/packages/webpack-cli/lib/utils/cli-flags.js
@@ -191,7 +191,7 @@ module.exports = {
             usage: '--dev',
             alias: 'd',
             type: Boolean,
-            defaultValue: undefined,
+            defaultValue: false,
             group: ZERO_CONFIG_GROUP,
             description: 'Run development build',
             link: 'https://webpack.js.org/concepts/#mode',
@@ -201,7 +201,7 @@ module.exports = {
             alias: 'p',
             usage: '--prod',
             type: Boolean,
-            defaultValue: undefined,
+            defaultValue: false,
             group: ZERO_CONFIG_GROUP,
             description: 'Run production build',
             link: 'https://webpack.js.org/concepts/#mode',
diff --git a/test/defaults/output-defaults.test.js b/test/defaults/output-defaults.test.js
index cceb480b8cf..11079b0f90e 100644
--- a/test/defaults/output-defaults.test.js
+++ b/test/defaults/output-defaults.test.js
@@ -6,8 +6,8 @@ const { run } = require('../utils/test-utils');
 describe('output flag defaults', () => {
     it('should create default file for a given directory', (done) => {
         const { stdout } = run(__dirname, ['--entry', './a.js', '--output', './binary'], false);
-        // Should print a warning about config fallback
-        expect(stdout).toContain('option has not been set, webpack will fallback to');
+        // Should not print warning about config fallback, as we have production as default
+        expect(stdout).not.toContain('option has not been set, webpack will fallback to');
         stat(resolve(__dirname, './binary/main.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
diff --git a/test/merge/config/merge-config.test.js b/test/merge/config/merge-config.test.js
index 8b9185ca8e4..80d2976865c 100644
--- a/test/merge/config/merge-config.test.js
+++ b/test/merge/config/merge-config.test.js
@@ -8,7 +8,7 @@ const { run } = require('../../utils/test-utils');
 describe('merge flag configuration', () => {
     it('merges two configurations together', (done) => {
         const { stdout } = run(__dirname, ['--config', './1.js', '--merge', './2.js'], false);
-        expect(stdout).toContain('option has not been set, webpack will fallback to');
+        expect(stdout).not.toContain('option has not been set, webpack will fallback to');
         stat(resolve(__dirname, './dist/merged.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
diff --git a/test/merge/defaults/merge-defaults.test.js b/test/merge/defaults/merge-defaults.test.js
index 7cf845f58ba..2b7a762c50c 100644
--- a/test/merge/defaults/merge-defaults.test.js
+++ b/test/merge/defaults/merge-defaults.test.js
@@ -1,6 +1,6 @@
 'use strict';
 
-const { stat } = require('fs');
+const { stat, readFile } = require('fs');
 const { resolve } = require('path');
 
 const { run } = require('../../utils/test-utils');
@@ -8,20 +8,32 @@ const { run } = require('../../utils/test-utils');
 describe('merge flag defaults', () => {
     it('merges a default webpack.base.config with default config lookup', (done) => {
         const { stdout } = run(__dirname, ['-m', './'], false);
-        expect(stdout).toContain('option has not been set, webpack will fallback to');
+        expect(stdout).not.toContain('option has not been set, webpack will fallback to');
+
         stat(resolve(__dirname, './dist/default.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
             done();
         });
+        readFile(resolve(__dirname, './dist/default.js'), 'utf-8', (err, data) => {
+            expect(err).toBe(null);
+            expect(data).toContain('some_entry');
+            done();
+        });
     });
     it('merges a configuration file with default base config', (done) => {
         const { stdout } = run(__dirname, ['-c', './1.js'], false);
-        expect(stdout).toContain('option has not been set, webpack will fallback to');
+
+        expect(stdout).not.toContain('option has not been set, webpack will fallback to');
         stat(resolve(__dirname, './dist/default.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
             done();
         });
+        readFile(resolve(__dirname, './dist/default.js'), 'utf-8', (err, data) => {
+            expect(err).toBe(null);
+            expect(data).toContain('some_entry');
+            done();
+        });
     });
 });
diff --git a/test/merge/defaults/some_entry.js b/test/merge/defaults/some_entry.js
index aedc30b9978..95556096294 100644
--- a/test/merge/defaults/some_entry.js
+++ b/test/merge/defaults/some_entry.js
@@ -1 +1 @@
-console.log('<3');
+console.log('some_entry');
diff --git a/test/merge/defaults/some_other_entry.js b/test/merge/defaults/some_other_entry.js
index bfaee7fc3d9..289750fec7e 100644
--- a/test/merge/defaults/some_other_entry.js
+++ b/test/merge/defaults/some_other_entry.js
@@ -1 +1 @@
-console.log('moo');
+console.log('some_other_entry');
diff --git a/test/mode/mode-single-arg/mode-single-arg.test.js b/test/mode/mode-single-arg/mode-single-arg.test.js
index c5282c653ae..1976447de82 100644
--- a/test/mode/mode-single-arg/mode-single-arg.test.js
+++ b/test/mode/mode-single-arg/mode-single-arg.test.js
@@ -3,10 +3,27 @@ const { run } = require('../../utils/test-utils');
 const { stat, readFile } = require('fs');
 const { resolve } = require('path');
 describe('mode flags', () => {
+    it('should set mode=production by default', (done) => {
+        const { stderr, stdout } = run(__dirname);
+        expect(stderr).toBeFalsy();
+        expect(stdout).toContain(`mode: 'production'`);
+
+        stat(resolve(__dirname, './bin/main.js'), (err, stats) => {
+            expect(err).toBe(null);
+            expect(stats.isFile()).toBe(true);
+            done();
+        });
+        readFile(resolve(__dirname, './bin/main.js'), 'utf-8', (err, data) => {
+            expect(err).toBe(null);
+            expect(data).toContain('production test');
+            done();
+        });
+    });
+
     it('should load a development config when --mode=development is passed', (done) => {
         const { stderr, stdout } = run(__dirname, ['--mode', 'development']);
         expect(stderr).toBeFalsy();
-        expect(stdout).toBeTruthy();
+        expect(stdout).toContain(`mode: 'development'`);
 
         stat(resolve(__dirname, './bin/main.js'), (err, stats) => {
             expect(err).toBe(null);
@@ -18,19 +35,24 @@ describe('mode flags', () => {
     it('should load a production config when --mode=production is passed', (done) => {
         const { stderr, stdout } = run(__dirname, ['--mode', 'production']);
         expect(stderr).toBeFalsy();
-        expect(stdout).toBeTruthy();
+        expect(stdout).toContain(`mode: 'production'`);
 
         stat(resolve(__dirname, './bin/main.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
             done();
         });
+        readFile(resolve(__dirname, './bin/main.js'), 'utf-8', (err, data) => {
+            expect(err).toBe(null);
+            expect(data).toContain('production test');
+            done();
+        });
     });
 
     it('should load a none config when --mode=none is passed', (done) => {
         const { stderr, stdout } = run(__dirname, ['--mode', 'none']);
         expect(stderr).toBeFalsy();
-        expect(stdout).toBeTruthy();
+        expect(stdout).toContain(`mode: 'none'`);
 
         stat(resolve(__dirname, './bin/main.js'), (err, stats) => {
             expect(err).toBe(null);
@@ -42,7 +64,8 @@ describe('mode flags', () => {
     it('should show warning and load a production config when --mode=abcd is passed', (done) => {
         const { stderr, stdout } = run(__dirname, ['--mode', 'abcd']);
         expect(stderr).toContain('invalid value for "mode" option. Using "production" by default');
-        expect(stdout).toBeTruthy();
+        expect(stdout).toContain(`mode: 'production'`);
+
         stat(resolve(__dirname, './bin/main.js'), (err, stats) => {
             expect(err).toBe(null);
             expect(stats.isFile()).toBe(true);
diff --git a/test/mode/mode-single-arg/webpack.config.js b/test/mode/mode-single-arg/webpack.config.js
new file mode 100644
index 00000000000..b1dcc533e36
--- /dev/null
+++ b/test/mode/mode-single-arg/webpack.config.js
@@ -0,0 +1,7 @@
+// eslint-disable-next-line node/no-unpublished-require
+const WebpackCLITestPlugin = require('../../utils/webpack-cli-test-plugin');
+
+module.exports = {
+    entry: './src/index.js',
+    plugins: [new WebpackCLITestPlugin()],
+};