In 1971, Halldorson turned pro. He joined the Canadian Tour in 1973 and the PGA Tour in 1975.
Halldorson had seven career wins on the Canadian Tour and its predecessors. He won one official PGA Tour event, the 1980 Pensacola Open, and finished a career best 36th on the PGA Tour money list that year. He won the unofficial Deposit Guaranty Golf Classic in 1986. Halldorson was a member of seven WGC-World Cup Canadian national teams (1976, 1978, 1979, 1980, 1982, 1985, 1991) including two winning teams (1980, 1985).
Halldorson played briefly on the Champions Tour after turning 50 in 2002. He operated a golf course design business with fellow former PGA Tour golfer Mike Morley. Halldorson also authored a golf instructional book and was the deputy director of the Canadian Tour