diff --git a/landing/header.css b/landing/header.css
--- a/landing/header.css
+++ b/landing/header.css
@@ -116,6 +116,7 @@
   }
 }
 
+/* max-width should be kept in sync with HEADER_BREAKPOINT in header.react */
 @media screen and (max-width: 848px) {
   .pageNav {
     display: none;
@@ -144,6 +145,7 @@
   }
 }
 
+/* This value should be kept in sync with HEADER_BREAKPOINT in header.react */
 @media screen and (max-width: 848px) and (hover: hover) {
   div.menuIcon:hover {
     cursor: pointer;
diff --git a/landing/header.react.js b/landing/header.react.js
--- a/landing/header.react.js
+++ b/landing/header.react.js
@@ -16,6 +16,10 @@
 import css from './header.css';
 import typography from './typography.css';
 
+// This value comes from the breakpoint value in header.css. Please make sure
+// that this value is in sync with header.css if ever changed
+export const HEADER_BREAKPOINT = 848; // px
+
 type Props = {
   +showMobileNav: boolean,
   +setShowMobileNav: SetState<boolean>,
diff --git a/landing/landing.react.js b/landing/landing.react.js
--- a/landing/landing.react.js
+++ b/landing/landing.react.js
@@ -11,7 +11,7 @@
 
 import AppLanding from './app-landing.react.js';
 import Footer from './footer.react.js';
-import Header from './header.react.js';
+import Header, { HEADER_BREAKPOINT } from './header.react.js';
 import Investors from './investors.react.js';
 import Keyservers from './keyservers.react.js';
 import css from './landing.css';
@@ -51,6 +51,19 @@
 
   const [showMobileNav, setShowMobileNav] = React.useState<boolean>(false);
 
+  React.useEffect(() => {
+    const handleResize = () => {
+      if (window.innerWidth > HEADER_BREAKPOINT && showMobileNav) {
+        setShowMobileNav(false);
+      }
+    };
+
+    window.addEventListener('resize', handleResize);
+    return () => {
+      window.removeEventListener('resize', handleResize);
+    };
+  }, [showMobileNav, setShowMobileNav]);
+
   const innerContainerClassName = classNames({
     [css.innerContainer]: true,
     [css.innerContainerMobileNav]: showMobileNav,