Ms Motion is moving to the chairman role from today, having spent two decades with the law firm.
She replaces Fred Tyler who is stepping down after eight years while Ms Motion's previous role as head of litigation is being filled by David Short.
Ms Motion will now lead a board made up of the heads of litigation, private client and commercial departments and one other elected member.
She said: "Our performance has been exceptionally good during challenging times, with various teams in the firm developing strongly. One of my key roles will be to raise our profile to let people know what Balfour + Manson is really about."
The most recent annual accounts for the firm, which cover the 12 months to the end of October 2012, show turnover of almost £8.1 million and operating profit of £3.13m.
The firm, which was founded in 1888 and employs 127 staff, has its main office in Edinburgh and another one in Aberdeen.
Mr Tyler, 62, said: "The time was right to step down and let others come through. The firm is in great shape and I am convinced that Elaine will be an excellent leader who will continue to drive Balfour + Manson forward."
Meanwhile, Brodies has hired two partners from rival Maclay Murray & Spens to expand its private client team.
Norman Kennedy and Alan Eccles both join the Glasgow office of the firm.