Brent Casselman began his major league officiating career as a lineman in the NHL in 1966, breaking in at a time when the NHL had only five referees and four full-time linesmen. When the WHA began play in 1972, Casselman joined fellow referees Bill Friday, Bob Sloan, and Ron Ego and jumpeed to the rival league, where he worked for three seasons. Following the 1974-1975 season, Casselman retired from officiating, honoring a promise he made to his wife to quit after the birth of their daughter. He moved on to become a sales manager for an automobile dealership in Hamilton, Ontario.
As a defenseman in junior hockey with the St. Catharines Teepees, Casselman's teammates included future NHL players Roger Crozier, Vic Hadfield, Dennis Hull, Doug Jarrett and Ray Cullen, in addition to future NHL and WHA star Pat Stapleton.