On 7 July 2009, Gustavsson ended weeks of speculation by signing a one-year contract for US $810,000 (plus a $90,000 signing bonus) with the Toronto Maple Leafs.

In an interview with the Toronto Sun, Gustavsson said, "I'm going to try to steal as many games from Vesa Toskala as I can, I'm a goalie and I want to play... I know the best goalie gets the ice time and if I play good I'm going to get the chance.
We will be good teammates and try to push ourselves, but my goal is to be the number one goalie... For the most part, I like [the attention].
It doesn't matter what people say about me, I know if I play good or if I play bad... I like when lots of people are getting involved.
In Toronto, everybody talks about hockey.
I like to be in the heat."

Gustavsson endured a setback on the first day of camp, traced to a heart condition that required an ablation surgery process.
Despite the setback, Gustavsson made his NHL debut to open the 2009–10 season with Toronto on 3 October 2009, against the Washington Capitals.
On 26 October, Gustavsson recorded his first NHL win against the Anaheim Ducks, making 25 saves on 28 shots in a 6–3 win.

After overcoming a groin strain, on 1 December, Gustavsson left after the first period in a game against the Montreal Canadiens with a heart murmur, which led to a second Radio Frequency ablation to treat a reentrant tachycardia.
After he was cleared to return, he responded by recording his first NHL shutout in a 2–0 win over the Boston Bruins on 19 December.
He won seven straight starts in March to tie the Maple Leafs’ club record for the longest winning streak by a rookie goaltender.
Gustavsson finished the season with a 16–15–9 record with a 2.87 GAA and a .902 save percentage.

On 6 April 2010, the Toronto chapter of the Professional Hockey Writers Association announced Gustavsson as the Maple Leafs nominee for the 2010 Bill Masterton Trophy, awarded for "perseverance, sportsmanship and dedication to hockey."

After becoming a restricted free agent in the summer, Gustavsson agreed a two-year extension with the Leafs on 15 April 2010, with an average salary of $1.35 million per year.

In the following 2010–11 season, on 4 February 2011, he was sent down to the Toronto Marlies, the American Hockey League (AHL) affiliate farm team of the Maple Leafs, for a two-week conditioning stint.
On 9 February, Gustavsson was pulled after the first period in a Marlies home game against the Connecticut Whale due to an accelerated heart beat.
Two days later, on 11 February, Gustavsson underwent a third minor heart ablation surgery and was placed on injured reserve by the Maple Leafs, suspending his conditioning stint.
Upon recovering from the surgery, Gustavsson resumed his conditioning stint with the Marlies on 22 February.
On 7 July 2009, Gustavsson ended weeks of speculation by signing a one-year contract for US $810,000 (plus a $90,000 signing bonus) with the Toronto Maple Leafs.
In an interview with the Toronto Sun, Gustavsson said, "I'm going to try to steal as many games from Vesa Toskala as I can, I'm a goalie and I want to play... I know the best goalie gets the ice time and if I play good I'm going to get the chance. We will be good teammates and try to push ourselves, but my goal is to be the number one goalie... For the most part, I like [the attention]. It doesn't matter what people say about me, I know if I play good or if I play bad... I like when lots of people are getting involved. In Toronto, everybody talks about hockey. I like to be in the heat."
Gustavsson endured a setback on the first day of camp, traced to a heart condition that required an ablation surgery process. Despite the setback, Gustavsson made his NHL debut to open the 2009–10 season with Toronto on 3 October 2009, against the Washington Capitals. On 26 October, Gustavsson recorded his first NHL win against the Anaheim Ducks, making 25 saves on 28 shots in a 6–3 win.
After overcoming a groin strain, on 1 December, Gustavsson left after the first period in a game against the Montreal Canadiens with a heart murmur, which led to a second Radio Frequency ablation to treat a reentrant tachycardia. After he was cleared to return, he responded by recording his first NHL shutout in a 2–0 win over the Boston Bruins on 19 December. He won seven straight starts in March to tie the Maple Leafs’ club record for the longest winning streak by a rookie goaltender. Gustavsson finished the season with a 16–15–9 record with a 2.87 GAA and a .902 save percentage.
On 6 April 2010, the Toronto chapter of the Professional Hockey Writers Association announced Gustavsson as the Maple Leafs nominee for the 2010 Bill Masterton Trophy, awarded for "perseverance, sportsmanship and dedication to hockey."
After becoming a restricted free agent in the summer, Gustavsson agreed a two-year extension with the Leafs on 15 April 2010, with an average salary of $1.35 million per year.
In the following 2010–11 season, on 4 February 2011, he was sent down to the Toronto Marlies, the American Hockey League (AHL) affiliate farm team of the Maple Leafs, for a two-week conditioning stint. On 9 February, Gustavsson was pulled after the first period in a Marlies home game against the Connecticut Whale due to an accelerated heart beat. Two days later, on 11 February, Gustavsson underwent a third minor heart ablation surgery and was placed on injured reserve by the Maple Leafs, suspending his conditioning stint. Upon recovering from the surgery, Gustavsson resumed his conditioning stint with the Marlies on 22 February.
Jonas Gustavsson