McKinnon's sacking had been widely reported since an emergency board meeting was held after Saturday's 2-0 home defeat by Inverness, but the club waited until Tuesday to make it official.
The second consecutive defeat left United fourth in the Ladbrokes Championship and five points adrift of leaders St Mirren.
A club statement said: "Dundee United and Ray McKinnon have mutually agreed the manager's departure with immediate effect
The club thanks Ray for his service and will now commence the search for a new manager."
Source : PA