Consecutive defeats against Livingston and then at home to Inverness Caley Thistle, yesterday, that leave United in fourth place in the Championship after ten games have prompted the move.
United will now start the search for their fourth boss in just over two years, though it’s not known if the new man will be in place in time for Saturday’s league visit to Dumbarton.
It’s also not clear yet if assistant manager Laurie Ellis will join McKinnon in leaving.
He arrived at the club along with the manager when he was appointed in May last year.
Names being mentioned in connection with taking charge of team affairs include former players Jim McIntyre and Steven Pressley, while 2010 Scottish Cup winning boss Peter Houston could also be in the frame.