[2] In 2000, Hemský moved to North America to play for the Hull Olympiques of the Quebec Major Junior Hockey League (QMJHL).

Pardubice won the 2004–05 Czech Extraliga for the first time in 16 years, and Hemský was named playoff most valuable player for his efforts.

During the season, on 4 January 2007, Hemský managed to tie a game against the Dallas Stars after Patrik Štefan fell in front of the Oilers' empty net, turning the puck over to Edmonton.

After receiving a stretch pass from Ryan Smyth, Hemský managed to slip the puck into the Stars net with two seconds remaining on the clock.

Hemský recorded seven goals and 15 assists during the first 22 games of the 2009–10 season before suffering a left shoulder injury following a check from behind by Los Angeles Kings forward Michal Handzuš.

Hemský, Taylor Hall, and Sam Gagner, led the Oilers with 42 points for the majority of the season, until the trio of injured players were surpassed by Jordan Eberle.

After missing the first part of the 2011–12 season due to injury, Hemský struggled, posting just 11 points in his first 25 games, including a nine-game pointless streak.

He picked up the pace thereafter, scoring 15 points in his next 22 games, leading the Oilers to sign him to a two-year, $10 million deal on 24 February 2012.

On 21 March 2012, Hemský recorded his first career NHL hat trick, finishing with four points in a 6–3 victory over the Nashville Predators.

After coming back from two shoulder surgeries, Hemský had a disappointing start to the season and finished the year with 36 points in 69 games.

He quickly found success on Ottawa's top line alongside center Jason Spezza,[7] recording six points in his first three games.

[17] In the 2008 tournament, Hemský tied for fourth place in scoring with six goals and five assists while leading the Czech Republic to a fifth-place finish.

[18][19] On 13 December 2017, Hemský, along with teammates Jordie Benn, Andrew Shaw, Karl Alzner, Jeff Petry, Charles Hudon, and Artturi Lehkonen, all went bald as part of the Leucan Shaved Head Challenge, as well as raised $33,000 for children battling cancer.