Selected by the Pittsburgh Penguins with the No. 22 pick in the 2014 NHL Draft, Kapanen has 54 points (28 goals, 26 assists) in 133 regular-season games with Toronto, and five points (four goals, one assist) in 20 playoff games.
He was acquired by Toronto on July 1, 2015, in a trade that sent forward Phil Kessel to Pittsburgh.
"Kasperi has been with us four years," Maple Leafs general manager Kyle Dubas said, "and [it's been great] to watch him grow as a person and player to last year, where he finally cracked the team out of training camp and played full time. He was outstanding on the penalty kill, used his speed, moved up and down the lineup, whatever we asked of him, he was great."
Johnsson, a 24-year-old forward, had 43 points (20 goals, 23 assists) in 73 games with the Maple Leafs this season. He had four points (one goal, three assists) in seven playoff games.
Selected by Toronto in the seventh round (No. 202) of the 2013 NHL Draft, Johnsson has 46 points (22 goals, 24 assists) in 82 regular-season games and six points (two goals, four assists) in 13 playoff games.