After playing with different junior teams, Komarov began his professional career in the Finnish SM-liiga with Ässät.
After winning a silver medal with Ässät in 2006, he moved to Pelicans.
After his season with Ässät, he was chosen in the sixth round of the 2006 NHL Entry Draft, 180th overall, by the Toronto Maple Leafs.

Komarov captained Finland at the 2007 World Junior Ice Hockey Championships in Leksand and Mora, Sweden.

During the 2009–10 season, Komarov played with Dynamo Moscow of the Kontinental Hockey League (KHL).

Komarov won a gold medal with Finland at the 2011 IIHF World Championships in Bratislava and Košice, Slovakia.

In May 2012, Komarov agreed to a contract with the Toronto Maple Leafs for the 2012–13 season.
The reported value of his contract was $1.2 million, including bonuses.
He played his first career NHL game on 19 January 2013, and he scored his first NHL goal (a game winner) against the Montreal Canadiens on 9 February 2013.

After one season with the Maple Leafs, Komarov left the NHL to sign a one-year contract to return with Dynamo Moscow in June 2013, with the ambition to make the Finnish 2014 Winter Olympic squad in a more offensive role with Dynamo; he made the roster, also serving as an alternate captain.

Despite signing with Dynamo Moscow, Komarov said he was not done with the NHL, and one year later, on 1 July 2014, he signed a four-year deal worth $2.9 million per season to return to the Maple Leafs.
Komarov played well following the signing, recording 4 goals and 16 assists in his first 23 games of the season, as well as 99 hits, enough for third in the NHL as of late November.
However, a blindside hit from former Dynamo Moscow linemate Alexander Ovechkin on November 29 left Komarov with concussion issues, and he would score at a lackluster pace for the rest of the season.

Komarov would rebound the next season and excel under new head coach Mike Babcock.
Komarov, who previously played on Toronto's second, third lines and fourth lines, was now experiencing top line duties (taking Phil Kessel's spot), and occasionally reprising his role on the second.
He led the Maple Leafs in goals and points up to the Christmas break.
Shortly after, he was named to the 2016 NHL All-Star Game on 6 January 2016.
Komarov had 27 points in 37 games when named an All-Star, including 15 goals, and was one point behind teammate James van Riemsdyk for the team lead in scoring.
Given Komarov's reputation and traditional NHL role as a bottom six grinder, he had not expected to be named an All Star, going as far as to book a vacation during the All-Star break, which was cancelled after he had been named to the festivities.
Komarov's production declined in the latter half of the season, partially due to the loss of van Riemsdyk in January to injury.
Despite this, Komarov finished the year third in team scoring with 36 points, behind only P. A. Parenteau and Nazem Kadri.
The following season, he shared a line with Kadri and rookie winger Connor Brown.
In 2017, Komarov played in the NHL Centennial Classic where he scored one goal in Toronto's 5–4 overtime win against the Detroit Red Wings.
In 2018, Komarov played in the 2018 NHL Stadium Series against the Washington Capitals, where the Leafs lost 2–5.

On 1 July 2018, Komarov signed a four-year contract with the New York Islanders.
After playing with different junior teams, Komarov began his professional career in the Finnish SM-liiga with Ässät. After winning a silver medal with Ässät in 2006, he moved to Pelicans. After his season with Ässät, he was chosen in the sixth round of the 2006 NHL Entry Draft, 180th overall, by the Toronto Maple Leafs.
Komarov captained Finland at the 2007 World Junior Ice Hockey Championships in Leksand and Mora, Sweden.
During the 2009–10 season, Komarov played with Dynamo Moscow of the Kontinental Hockey League (KHL).
Komarov won a gold medal with Finland at the 2011 IIHF World Championships in Bratislava and Košice, Slovakia.
In May 2012, Komarov agreed to a contract with the Toronto Maple Leafs for the 2012–13 season. The reported value of his contract was $1.2 million, including bonuses. He played his first career NHL game on 19 January 2013, and he scored his first NHL goal (a game winner) against the Montreal Canadiens on 9 February 2013.
After one season with the Maple Leafs, Komarov left the NHL to sign a one-year contract to return with Dynamo Moscow in June 2013, with the ambition to make the Finnish 2014 Winter Olympic squad in a more offensive role with Dynamo; he made the roster, also serving as an alternate captain.
Despite signing with Dynamo Moscow, Komarov said he was not done with the NHL, and one year later, on 1 July 2014, he signed a four-year deal worth $2.9 million per season to return to the Maple Leafs. Komarov played well following the signing, recording 4 goals and 16 assists in his first 23 games of the season, as well as 99 hits, enough for third in the NHL as of late November. However, a blindside hit from former Dynamo Moscow linemate Alexander Ovechkin on November 29 left Komarov with concussion issues, and he would score at a lackluster pace for the rest of the season.
Komarov would rebound the next season and excel under new head coach Mike Babcock. Komarov, who previously played on Toronto's second, third lines and fourth lines, was now experiencing top line duties (taking Phil Kessel's spot), and occasionally reprising his role on the second. He led the Maple Leafs in goals and points up to the Christmas break. Shortly after, he was named to the 2016 NHL All-Star Game on 6 January 2016. Komarov had 27 points in 37 games when named an All-Star, including 15 goals, and was one point behind teammate James van Riemsdyk for the team lead in scoring. Given Komarov's reputation and traditional NHL role as a bottom six grinder, he had not expected to be named an All Star, going as far as to book a vacation during the All-Star break, which was cancelled after he had been named to the festivities. Komarov's production declined in the latter half of the season, partially due to the loss of van Riemsdyk in January to injury. Despite this, Komarov finished the year third in team scoring with 36 points, behind only P. A. Parenteau and Nazem Kadri. The following season, he shared a line with Kadri and rookie winger Connor Brown. In 2017, Komarov played in the NHL Centennial Classic where he scored one goal in Toronto's 5–4 overtime win against the Detroit Red Wings. In 2018, Komarov played in the 2018 NHL Stadium Series against the Washington Capitals, where the Leafs lost 2–5.
On 1 July 2018, Komarov signed a four-year contract with the New York Islanders.
Leo Komarov