 He was general counsel and vice president for Investors Mutual Funds of Minneapolis from 1961 to 1969.During his service as United States attorney, MacKinnon successfully prosecuted the first labor case under the Taft-Hartley Act against Archer Daniels Midland, Gerald Connelly, and Sidney Brennan in "United States v. Gerald Connelly".During his service as Special Assistant United States Attorney General in 1960, MacKinnon focused on labor racketeering investigations involving James Hoffa.