[1] Turning professional in the 2001–02 season, Clemmensen made his NHL debut in New Jersey's season-opening 6–1 loss to the Washington Capitals,[2] and played in two games for the Devils, while playing the majority of his professional rookie season with the Albany River Rats of the American Hockey League (AHL), the Devils' minor league affiliate.

[4] Following the 2004–05 NHL lockout, Clemmensen became Martin Brodeur's permanent backup, appearing in 13 games (starting in nine) and posting a 3.35 GAA during the 2005–06 season.

He made his first start with the Maple Leafs on January 1, 2008, as the result of an injury to starter Vesa Toskala, and recorded a 4–3 shootout win against the Tampa Bay Lightning.

[6] Going into the 2007 Calder Cup playoffs back with the Marlies, Clemmensen assumed the starting role over Pogge and helped the club to the Western Conference semifinals against the Chicago Wolves, where they were eliminated in five games.

His 25 wins are the highest total by a goaltender besides Brodeur in franchise history, while his 2.39 GAA and .917 save percentage were statistically in the league's top ten.

[12] On November 3, 2011, the Panthers loaned Clemmensen, who suffered a knee injury in the pre-season, to the San Antonio Rampage of the AHL for conditioning purposes.

[citation needed] He eventually reunited with former Devils teammate, John Madden in 2012, helping the Panthers clinch the playoffs for the first time in 12 years.