Introduced on October 23, 1999, it was promoted by Apple as "The Best Internet Operating System Ever",[5] highlighting Sherlock 2’s Internet search capabilities, integration with Apple's free online services known as iTools and improved Open Transport networking.

While Mac OS 9 lacks protected memory and full pre-emptive multitasking,[6] lasting improvements include the introduction of an automated Software Update engine and support for multiple users.

At the 2002 Worldwide Developers Conference, Steve Jobs began his keynote address by staging a mock funeral for OS 9.

Mac OS 9 also featured integrated support for Apple's suite of Internet services known as iTools (later re-branded as .Mac, then MobileMe, which was replaced by iCloud) and included improved TCP/IP functionality with Open Transport 2.5.

Some Mac OS 9 applications do not run well in Classic; they demonstrate screen redraw problems and lagging performance.

Initially, it required an actual PowerPC processor present in the machine it was running on similar to a hypervisor, but support for x86 platforms was added at a later date.

SheepShaver cannot run Mac OS versions newer than 9.0.4, however, as it does not have support for a memory management unit.

Mac OS 9.0.4 was a collection of bug fixes primarily relating to USB and FireWire support.