The software has gone into so-called 'Gold Master' version, meaning the release is almost identical to the one that will roll out to the public at the end of this month.
Apple has not yet put a precise date on the consumer release of Mountain Lion, but has said that it will cost just $19.99 (£12.80).
As Apple has an earnings call for its second quarter results on July 24, it is expected that the firm will release the software on that day, or close to it.
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.
Headline features in the software include greater 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.
A big new feature is Gatekeeper, which 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.
Whilst the feature is claimed to offer people more security around the apps they use, it is also being viewed as a way for Apple to push users towards its 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.
Mountain Lion also introduces the Notification Centre, 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, social network Twitter is integrated into Mountain Lion, making it easier to share web links, photos and other content.
Apple said that Facebook integration will be available in an upcoming software update to Mountain Lion, presumably 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 will be available later this month from the Mac App Store for $19.99. It will require 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.
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