Lemaire, 69, spent parts of seven seasons as coach of the New Jersey Devils under president and general manager Lamoriello, who became GM of the Maple Leafs on July 23.
"I have been working with Lou for so many years and we became real good friends," Lemaire told the Maple Leafs website. "I love the work. Also, you've got [Maple Leafs coach] Mike [Babcock], I worked at the Olympics with him. I feel that he's the best coach around and it'll be a thrill to be able to assist him.
"That's what made me really excited, because Mike called me and Lou called me that same day and I feel that through the two calls, I said I think I can be working with them and bring something to the table."