Bill Mawhinney began to caddy as a youngster at Quilchena Golf Course. He won his first medal in 1946 and in the following few years won almost every city, provincial, and national title. In 1950, he was named Sport Monthly’s “Golfer of the Year” and was made a life member of the Quilchena Golf Club. In 1952, he turned pro, and was invited to many prestigious tournaments before retiring from the circuit in 1957.

Mawhinney won the BC Junior, the Vancouver City Junior, and the BCGA Junior Match Play titles in 1947 and 1948. He won the BC Closed Amateur in 1947-48, 1950-52.

In the Vancouver Province Match Play Open, Mawhinney defeated fellow BC Sports Hall of Famers Stan Leonard and Walt McElroy in 1947 and 1949 respectively. In 1948, he won the Canadian Junior title and was a runner-up at the 1952 Trail Open. He also won four Vancouver City amateur titles from 1949-52.

Mawhinney was a member and low medalist of the Willingdon Cup team from 1949-52 and played a major part in the team’s 1952 victory. He was also a member of the Morse (1949-52)and Hudson (1950-52) Cup teams, and won two matches himself during that period. Mawhinney also stood as a member of the Royal Canadian Golf Association in 1951 and 1952 America’s Cup teams.

In 1950, he won the Canadian amateur championship at St. John, New Brunswick. He was a semi-finalist at the 1950 Pacific Northwest Golf Association tournament. He also won the New Westminster amateur championships from 1950-52 and the BC amateur championship in 1951-52.

Mawhinney also played successfully farther afield. He competed in the 1951-53 Masters at Augusta, Georgia, and the 1951 National Pro-Am championship at Pebble Beach. He finished as low amateur at the Wilmington Azalea Open in Cape Fear and competed at the 1951 Phoenix Open.

In 1952, Mawhinney won the Pacific Northwest Amateur in Tacoma, finished as a semi-finalist at the US Amateur in Seattle, and was a medalist at the Canadian Amateur. That same year he won the Tacoma and Seattle Opens. In 1953, he won the Powell River Open, 1953 and the Ogopogo Open in Kelowna from 1954-56.

He finished as low score medalist in regional qualifying rounds of the 1955 and 1959 US Opens. He was also a member of the 1956 CPGA International Golf Trophy team. In 1957, he won the BC Open, the Gaylord PGA championship, and the Canadian Professional championship.