After a developer version of the software was unleashed last week, Mountain Lion is now available for Mac consumers to download, priced at $19.99 (£13.99 in the UK).
Apple chief executive Tim Cook announced the news alongside the company's latest quarterly results yesterday (July 24).
"We've... just updated the entire MacBook line, will release Mountain Lion tomorrow and will be launching iOS 6 this fall," said Cook. "We are also really looking forward to the amazing new products we've got in the pipeline."
Sales of Mac computers were up 2% on the year, but Apple's results fell below most analyst expectations, resulting in its share price dropping 5% in after-hours trading on Wall Street.
Announced earlier in the year and then previewed at Apple's WWDC last month, Mountain Lion brings more than 200 new features to Mac users.
These include closer integration of Apple's iCloud cloud storage solution, along with various features from the iOS mobile operating system, such as Twitter integration, Notification Centre and iMessages.
When users start up Mountain Lion for the first time they will be prompted to sign in with their Apple ID and then set up iCloud, video calling service FaceTime, the iTunes Store and Mac App Store.
Using an iCloud account, Mountain lion users can sync their mail, contacts, calendar, messages, reminders and notes to other devices, while Documents in the Cloud works with iCloud-enabled apps, including Apple's iWork suite, to offer instant access to files from anywhere with an internet connection.
The new iMessages app replaces iChat on the Mac, allowing users to send messages to anyone with an iPhone, iPad, iPod Touch or another Mac. The messages can include attachments and high-quality videos.
Gatekeeper allows users to get better control over the apps downloaded on their machine, including controls for just running apps downloaded from the Mac App Store, or ones with a "unique developer ID" produced by Apple.
Apple claims the feature offers people more security around the apps they use, but it is also being viewed as a way for Apple to push users towards its Mac App Store.
Mountain Lion brings Notification Centre to the Mac, meaning all notifications from OS X or third-party apps appear in one convenient place. Users can personalise the app notifications they receive and turn off notifications temporarily if they don't want to be disturbed.
Just like iOS on iPhone, iPad and iPod Touch, social network Twitter is integrated into Mountain Lion, enabling users to directly share web links, photos and other content on the microblogging service.
Apple said that Facebook integration will be available in an upcoming software update to Mountain Lion, presumably launched at the same time as it is included in iOS 6 later in the year.
Announcing Mountain Lion last month, Apple's senior vice president of marketing Phil Schiller said: "The pace of innovation on the Mac is amazing, OS X Mountain Lion comes just a year after the incredibly successful launch of Lion.
"With iCloud built right in and the new Notification Centre, Messages, Dictation, Facebook integration and more, this is the best OS X yet."
OS X Mountain Lion is available now to buy from the Mac App Store. It requires users to have Lion or Snow Leopard (OS X v10.6.8 or later) installed, along with 2GB of memory and 8GB of available space.
With its latest Mac operating system update, Apple has more closely aligned its OS X software with the iOS system that underpins its mobile devices.
However, it has stopped short of the same sort of total integration as Microsoft's Windows 8, which will launch in October and is optimised for both traditional and touchscreen computers.
Windows 8, with its touchscreen-enabled Metro interface, is being viewed as the biggest step forward for Microsoft's operating system in over a decade.
In contrast, Mountain Lion is considered just an update and refresh of OS X.
Purchase OS X Mountain Lion here.