Thornton finished last season with seven goals and 24 assists in 70 games for the Sharks.
The 41-year-old has played 22 seasons in the NHL. The Boston Bruins took him first overall in the 1997 NHL draft, and he proceeded to score 169 goals and 285 assists in seven-plus seasons with the B's, who traded him to the San Jose Sharks during the 2005-06 season. Thornton won the Hart Memorial Trophy and Art Ross Trophy that year.
He starred with the Sharks, too, posting 251 goals and 804 assists during his 15 seasons in San Jose, who made the playoffs 13 times and the Stanley Cup Final once during his tenure. The Sharks lost the Cup in six games to the Pittsburgh Penguins in 2016.
Thornton also won gold with Team Canada during the 2010 Winter Olympics and 2014 and 2016 World Cups.
Inking a deal with the Maple Leafs signals a return near home for Thornton. He is a native of London, Ontario, which is roughly 120 miles southwest of Toronto.