Which macOS version introduced the Notification Center?

The macOS version that introduced Notification Center was OS X Mountain Lion, version 10.8. Apple released Mountain Lion on July 25, 2012, bringing a centralized place to review alerts from applications.

Notification Center was inspired by the corresponding feature Apple had already introduced in iOS 5 in 2011. On the Mac, it collected notifications such as messages, calendar reminders, and application alerts in a sidebar, allowing users to review them even after temporary banners disappeared from the screen.

Mountain Lion was the ninth major release of macOS, then called OS X, and followed OS X Lion. It also introduced or expanded features including Messages replacing iChat, Reminders as a separate application, Game Center, Gatekeeper, and closer iCloud integration. A common mix-up is attributing Notification Center to OS X Mavericks or Yosemite, which changed and refined the feature but did not introduce it on the Mac.