MacKinnon was a member of the editorial board of the Minnesota Law Review and played as a center on the Golden Gophers football team.

[citation needed] During his service as Special Assistant United States Attorney General in 1960, MacKinnon focused on labor racketeering investigations involving James Hoffa.

MacKinnon started the Test Fleet investigation that led to James Hoffa's conviction for perjury.

[citation needed] MacKinnon was nominated by President Richard Nixon on April 23, 1969, to a seat on the United States Court of Appeals for the District of Columbia Circuit vacated by Judge Charles Fahy.

"[5] Chief Justice Warren E. Burger, a fellow Minnesotan who appointed MacKinnon to the United States Sentencing Commission and the special court division that selected independent counsel, said, "He's a man who can't be frightened or pushed by anybody" when describing him.