The Hugh Fraser Foundation, a long-term supporter of the school, has pledged £300,000 while the Cameron Mackintosh Foundation, which supports art projects, has committed £250,000.
Around £150,000 has also been given in individual donations since the fire, GSA said.
It adds to a UK Government pledge of £5 million and a Scottish Government commitment to match funds up to £5 million to support the restoration.
It is thought it will cost £20 million to properly restore the renowned Grade A-listed building. Around 10% of the structure of the building, including the unique library, was destroyed in the blaze in May.
The drive to restore the Mackintosh to its former glory has drawn support from former student Peter Capaldi and Mackintosh fan Brad Pitt.
The Hugh Fraser Foundation was established in 1960 by Sir Hugh Fraser with shares in House of Fraser.
Chairman Dr Kenneth Chrystie said: "The trustees of the Hugh Fraser Foundation were clear that we wished to make a major and public show of support for the school following the fire on May 23.
"We felt that it was a great privilege to be able to support the GSA to begin the process of reconstruction and recovery.
"I am confident that the fundraising appeal will attract worldwide support from those who share with us the desire to see the school not only recover, but regain momentum towards its future success."
Sir Cameron Mackintosh, founder of the foundation of his name, said he hoped the donations would encourage more people to get involved.
"Since I became a theatre producer I have taken great pleasure from having a namesake as illustrious as Charles Rennie Mackintosh," he said.
"His legacy as an artist and designer, and his unique style and invention, lived on through the amazing building he designed for The Glasgow School of Art. Successive generations of artists have been inspired and nurtured within its walls.
"My admiration for Charles Rennie Mackintosh and my personal passion for original architectural design makes me even more pleased to be able to become one of the first donors to your appeal."
Professor Tom Inns, director of the GSA, said the school was "hugely grateful" for the donations.