Brian McDonald played his major junior hockey with the powerful St. Catharines Blackhawks in 1963-64 and 1964-65 playing with the likes of Fred Stanfield, Dennis Hull, Ken Hodge, Doug Jarrett and Tom Reid, all of whom had successful NHL careers.
McDonald turned pro in 1965-66, joining the St. Louis Braves, where he played for three years before moving on to the Dallas Blackhawks in 1967. It was apparent the Chicago Blackhawks were not going to make use of McDonald as they already had an abundance of talent at the centre position. In September, 1970 he was traded to Denver of the WHL for cash.
In 1970-71 he was called up to play in 12 games with the expansion Buffalo Sabres, but it was just to fill in temporarily for other injured players. McDonald was held pointless, but he did spend 29 minutes admiring the game from the penalty box.
McDonald's big break came in 1972-73 with the WHA coming into existence. He was selected by the Dayton-Houston Aeros in the 1972 General Entry Draft and played with the Aeros in 1972-73. He was traded to the Los Angeles Sharks in 1973-74. He remained in the WHA for another three years playing for the Michigan Stags and Indianapolis Racers.