Toronto had been without a general manager since coach Pat Quinn gave up his dual role June 27. It was a joint decision made by Quinn and Maple Leafs Sports & Entertainment Ltd. CEO Richard Peddie.
Quinn wanted Vancouver Canucks director of player personnel Steve Tambellini, who has previously worked with him.
Former New York Rangers general manager Neil Smith and Hockey Canada president Bob Nicholson also were finalists for the position.
But the Leafs decided to make the 36-year-old Ferguson the youngest general manager in the NHL. He served as the director of hockey operations for the St. Louis Blues since 2001 after five years as the team's assistant GM, and ran their American Hockey League team in Worcester, Massachusetts.