East led the creation of ARM's Design Consulting and Development Tools business, and when it went public in 1998 he took up the role of vice president of Business Operations.
In 2001 East was appointed as chief executive of ARM and led it over the 12 years in which it became the world's elading semiconductor IP licensing company, with its processors dominating the smartphone market.
In welcoming the appointment, BT's chairman, Sir Michael Rake, said: "I am delighted that Warren will be joining BT. He brings with him in-depth experience of the technology industry which will be a real asset to the Board.
East added: "I'm really pleased to be joining BT at such an exciting time in its strategy execution and look forward to the opportunities ahead."