Activist investor Morfit Mason and ValueAct sold about 7 million shares of Microsoft's stock over the past week, according to regulatory filings released on Tuesday.
The shares were sold at prices ranging from $72.50 apiece to $72.61 apiece, valuing the stake at over $500 million.
Morfit sits on Microsoft's board of directors, a seat he got after ValueAct's campaign in 2013. The San Francisco-based investment company announced a $2 billion stake in Microsoft on CNBC, amid shareholder grumblings that the stock had not performed well under then-CEO Steve Ballmer.
Ballmer announced his resignation shortly before ValueAct was granted a board seat, although he said it was not related to pressure from ValueAct.
Shares of Microsoft have shot up nearly 125 percent since Ballmer announced his retirement in August 2013, when shares traded at prices between $30 and $35 a share. CEO Satya Nadella has pushed the company deeper into cloud computing and computer hardware, and made a splash acquisition of LinkedIn, expected to expand Microsoft's reach into industries like recruiting and human resources.
More recently, ValueAct, which manages about $15 billion, has turned its attention to companies like Valeant Pharmaceuticals, and also has stakes in various energy, healthcare and technology companies.
WATCH: MSFT officially lists A.I. as one of its top priorities
