Mellon, whose United team finished ninth in the Scottish Premiership last season, previously spent four years with Tranmere.
The club said the mutually agreed decision followed a "constructive and open discussion" about his tenure.
Jack Fitzwater and Peter Pawlett had their original sanctions re-imposed after the claims were rejected.
Dundee United Supporters' Foundation pass a proposal to transfer £60k to assist the club and raise a further £40k.
Club set to hold talks with the SFA and NHS Tayside to discuss coronavirus outbreak at Tannadice.
The Scotland international has moved to Tannadice on season-long loan.
The Dundee United defender will not face an SFA charge over a tackle that left Morelos injured.
The SPFL have announced the fixture list for the coming season.
Lord Clark ruled that the dispute over relegation should be dealt with by an SFA -convened panel.
The Scotland captain is an English league champion after Liverpool topped the table.
The Celtic boss says sport is secondary to the human cost of coronavirus.