Apple - What's the font in Mac OS X's GUI?

The font you see in most places on Mac OS X is Lucida Grande, it is the system font in OS X 10.1 Puma to 10.9 Mavericks. Mac OS 1-7 used Chicago, then Mac OS 8 and 9 used Charcoal.

The most recent additions are Helvetica Neue (OS X 10.10 Yosemite) and San Francisco (OS X 10.11 El Capitan through Mojave)

The font used from Mac OS X is "Lucida Grande"; in Mac OS 8 and 9, the font used for menus and window title was "Charcoal," but it could be changed from the preference panel.

From System 1 up to 7.6 the default GUI font was Chicago.

Since the release of OS X 10.11 (El Capitan) in 2014 the Mac system typeface family is San Francisco.

Type Detail's page on San Francisco

San Francisco is also used in iOS, WatchOS, and tvOS.