The move following months of lobbying by ValueAct for a say on how Microsoft is run. Under the agreement struck last summer, Morfit had the option of joining the board at the first quarterly board meeting after November's annual shareholder meeting.
ValueAct bought a $2 billion stake in Microsoft last April and immediately began agitating for change. In August, longstanding chief executive Steve Ballmer announced his retirement, which went into effect last month when insider Satya Nadella was named as CEO and co-founder Bill Gates stepped down as chairman.
Microsoft's board now has 11 members, including Ballmer and Gates.