The brightness keys are the kind of thing you press four hundred times a month without thinking, which is why it’s so jarring when a press does nothing. The causes are few, and they sort cleanly by how long the fix takes. Work down this list in order; most people are done by the second entry.
1. The function-key switch is flipped
If pressing F1 opens a help window, mutes a call, or does whatever the frontmost app decided F1 means, macOS is treating the top row as standard function keys. System Settings > Keyboard > Keyboard Shortcuts > Function Keys has the switch: “Use F1, F2, etc. keys as standard function keys.” Turn it off, or leave it on and hold fn while you press. This is the thirty-second fix, and it’s the most common cause on a Mac whose keys stopped working “out of nowhere” (usually right after installing a tool that asked for exactly this setting).
2. You’re looking at an external monitor
This one isn’t a malfunction, though it does a perfect impression of one. The brightness keys drive the displays macOS knows how to control: the built-in screen, and the handful of Apple-blessed externals. Plug in an ordinary monitor and the keys keep dutifully dimming the MacBook panel while the big screen ignores them. That’s not a fault; macOS simply has no built-in brightness control for most external monitors. The ways around that are their own guide, how to control external monitor brightness on a Mac. (For what it’s worth, this is also a thing Brightness Controller fixes directly: it can put the brightness keys in charge of every screen, not just the built-in one.)
3. The keyboard isn’t Apple’s
A third-party keyboard’s F1 is usually just F1. The sun-symbol behavior of a MacBook’s top row is something Apple keyboards send and others mostly don’t, so a mechanical keyboard that “broke” your brightness keys never had them. Check whether the keyboard’s own software (or a remapper like Karabiner-Elements) can map a key to “brightness up/down,” and note that some keyboards have an Fn-lock of their own that swaps the row’s meaning.
4. Another app claimed the keys
Anything that registers global shortcuts can sit in front of the brightness keys: window managers, screenshot tools, games running full screen, and, yes, display utilities. Several monitor-control apps deliberately intercept the brightness keys so they can forward the press to an external monitor; if one of them is misconfigured or half-quit, the press can vanish into it. Quit your menu bar apps one at a time and test after each. Whichever one restores the keys when it dies is your suspect.
5. The keys work, but the screen fights back
If the brightness visibly changes and then drifts back, the keys are fine and an automatic system is overriding you: “Automatically adjust brightness” in System Settings > Displays riding the ambient light sensor, or Low Power Mode capping the backlight on battery. That’s not a broken key, it’s a disagreement, and the general brightness guide covers who wins and how to make it you.
And if you’ve reached the bottom of the list with keys that genuinely do nothing on the built-in screen, with an Apple keyboard, with no interceptors running: restart before you diagnose further. The brightness engine in macOS very occasionally wedges, a reboot clears it, and no amount of settings-spelunking will.