The Station Park side confirmed on Tuesday that the assistant, who has also represented the club as a player, had relinquished his role.
Robertson spent the 2019/20 season as a player with Forfar Athletic at the end of a lengthy and celebrated playing career that saw him win a couple of Scotland caps.
He returned to the club in the assistant manager role a year ago, combining his part-time Station Park role with his full time-post as under 18 coach at Dundee FC.
In a statement released by Forfar Athletic, the club said: “This arrangement was possible thanks to the kind agreement of the Dens Park hierarchy and sanctioned by the SFA/SPFL.
"However, with the changes that have taken place at Dens Park over the past few weeks, Scott has taken on more enhanced role behind the scenes and has to give it his full commitment on a daily basis.
"Scott has been an integral part of our own backroom team throughout last season and we him every success moving forward at Dundee.
"At the same time, the Forfar Athletic directors would like to put on record their thanks to their Dundee counterparts for their co-operation over the past year.
"Gary (Irvine, manager) is currently looking at various options in regard to his own management team as training for the new campaign begins this coming weekend.”
Forfar have upcoming home friendlies against Dunfermline Athletic on Tuesday, June 28 and the Washington Huskies on Wednesday, July 6.