He played 13 seasons in the National Hockey League (NHL) with the Dallas Stars, Pittsburgh Penguins, Washington Capitals, and Philadelphia Flyers from 2007 to 2020.

[9] On March 19, 2007, he signed an amateur tryout contract with the Iowa Stars—the Dallas Stars' American Hockey League (AHL) affiliate at the time—for the remainder of the 2006–07 season.

On February 21, 2011, Niskanen was traded (along with teammate James Neal) to the Pittsburgh Penguins in exchange for defenseman Alex Goligoski.

[10] During the 2013–14 season, on March 4, 2014, Niskanen had his first multi-goal game with a two-goal effort in a 3–1 win over the Nashville Predators away at the Bridgestone Arena.

[11] In the final year of his contract with the Penguins and approaching free agency, Niskanen finished the season establishing new career highs in goals (10) and assists (36) for 46 points.

He recorded nine points and appeared in all 24 games of the playoffs while logging over 25 minutes of ice time to help the Washington Capitals claim their first Stanley Cup.

On June 14, 2019, with two-years still remaining on his contract, Niskanen was traded by the Capitals to the Philadelphia Flyers in exchange for defenceman Radko Gudas.

On February 21, Niskanen was fined $5,000 USD, the maximum allowable under the NHL Collective Bargaining Agreement, for a slashing penalty against Columbus Blue Jackets forward Gustav Nyquist.