He left the Dominions following the season for a senior team in Grand Forks, British Columbia, where he was the provincial league's leading goalscorer with 15 goals in 1913–14.

[1] Hoping to advance his career, MacKay wrote to Frank Patrick, operator of the Pacific Coast Hockey Association (PCHA), expressing his desire to turn professional.

[1] MacKay made his professional debut on December 8, playing on a line with Frank Nighbor and Ken Mallen and scored a hat trick in his first game.

[9] MacKay suffered a broken jaw and lost five teeth as a result,[10] while Wilson was later banned from the PCHA for life.

[13] MacKay returned to Vancouver and the professional ranks in 1920 amidst charges by Millionaires' owner Frank Patrick that the Big-4 was secretly paying players in violation of amateur standards.

[16] In two years with the Black Hawks, MacKay scored 31 goals before his playing rights were sold to the Pittsburgh Pirates prior to the 1928–29 NHL season.

[1] He appeared in only ten games with the Pirates before he was traded to the Boston Bruins on December 20, 1928 in exchange for Frank Fredrickson.

[18] He won the second Stanley Cup championship of his career that season as Boston defeated the New York Rangers to win the title.

"[1] His greatest limitation was his size, standing five foot nine and around 162 pounds; he frequently battled through injuries and missed many games throughout his career as a result.

[18][21] He contemplated purchasing the Spokane Clippers of the minor professional Pacific Coast Hockey League in 1938 but decided against taking over the team.

He died on May 30, 1940, when traveling as part of his mining job after his vehicle left the road and hit a telephone pole near the village of Ymir.