Local practices Page and Park Architects and ZM Architects kicked off the revamp on Monday (9 July) to restore the building to Mackintosh's original design - returning a number of the studio spaces to student use.
The two firms are heading the facelift more than a decade after the project was announced. The scheme has taken 10 years to secure funding and almost £1 million remains to be secured to complete the work.
Director of the Glasgow School of Art Seona Reid said: 'The Mackintosh building is not only home to one of the UK's leading international art schools - it is also a jewel in Scotland's cultural crown.
'We are delighted that we are now able to start work on the conservation of the building and our extensive collection and archive, making them accessible to as wide an audience as possible.'
Work will be carried out over three consecutive periods with completion set for 2009, the centenary of the Mackintosh building, which was built in two phases from 1896 to 1909.
A fourth and final phase is planned, which will see the creation of a new visitor centre opposite the Mackintosh building, providing a 'window on the Mac'.
Reid added: 'The new facilities for the Mackintosh Research Centre will significantly extend access to our breathtaking archives and collections, charting the development of art, design and architecture education over the past 150 years.
'It is also home to an extensive and often unseen collection of non-Mackintosh works of art, design and architecture - from 17th-century tapestries to the archive of Gillespie Kidd and Coia, one of the most influential firms of post-war architects.'