Selected 23rd overall by the Washington Capitals in the 1978 NHL Entry Draft, MacKinnon signed to play with the Winnipeg Jets of the World Hockey Association. He helped the team win the Avco World Trophy in 1978-79.
The Capitals reclaimed MacKinnon prior to the 1979–80 season when the Jets joined the NHL. He played with the Caps until the end of the 1983–84 NHL season. MacKinnon scored 5 goals and 23 assists in 147 NHL games.