Frögren was drafted in the eighth round, 206th overall, by the Calgary Flames in the 1998 NHL Entry Draft. Frögren is known for being a strong, solid defenceman. He is not renowned for his offensive ability, but is more suited to a stay-at-home style of defence. He has played recently with the Swedish national team at the 2008 World Championships.
Frögren began his hockey career by playing with Färjestad as a junior in the mid 1990s, and, except for a one-year loan to Bofors IK, played his entire Swedish professional career with Färjestad. He played seven seasons with Färjestad and has an impressive record, having won two Swedish Championships and four Swedish Championship silver medals. The only year that he did not play in the final of the playoffs was 1998–99, when Färjestad lost in the quarter-finals against Djurgårdens IF.
On July 9, 2008, Frögren signed a two-year contract with the Toronto Maple Leafs of the NHL. However, the contract Frögren signed was rejected by the NHL due to a dispute over his status. While the NHL believes he is an entry level player (as he was drafted by the Calgary Flames in 1998 but didn't sign with them), the Leafs, and Frögren's agent, Don Meehan, believe he is an unrestricted free agent, thus signing him to a contract worth more than the $875,000 limit that is mandated by the NHL for entry level players. Frögren has played for the Leafs under the terms of the original contract until the contract dispute between the NHL and its players' association is resolved.
On November 22, 2008, during a Maple Leafs' loss against the Chicago Blackhawks, Frögren suffered two muscle tears in his arm, and is expected to miss at least six weeks of game time due to the injury.