Lofthouse was a standout in the BCJHL with the Kelowna Buckaroos in 1973-74. He then played three years for the New Westminster Bruins where he produced 158 career goals. Lofthouse was chosen 21st overall by the Washington Capitals at the 1977 NHL Amateur Draft after helping the Bruins win the Memorial Cup and being named to the tournament all-star team.
The young forward split his first pro season, 1977-78, between the Caps, the AHL's Hershey Bears, and the Salt Lake Golden Eagles of the CHL. During the next two years, he was a regular with that club and scored a combined 28 goals playing with Dennis Hextall and Bobby Girard. He was relegated to Hershey in 1980-81 and responded with a league-leading 48 goals and 103 points. Lofthouse was presented the John B. Sollenberger Trophy for accumulating the most points in the league and was voted on to the AHL first all-star team.
In July 1981, he was sent to the Detroit Red Wings for goalie Al Jensen. He was a role player for two years with his new club then relegated to the AHL on a full-time basis. After scoring 101 points for the New Haven Nighthawks in 1983-84, he was placed on the league's second all-star team. Lofthouse was a top AHL player until he retired after scoring 32 goals for the Hershey Bears in 1988-89.