The Serb played for the Cottagers on loan last season, spearheading their promotion push with 12 goals.
Mitrovic has struggled to hold down a first-team place since Rafa Benitez arrived at Newcastle in 2016, meaning his exit has been on the cards for some time.
Newcastle last week rejected a £16m bid from Fulham for the 23-year-old.
However, the Magpies have now accepted a £20m offer for Mitrovic’s services.
The striker has left the club’s training camp in Portugal this evening, returning to London to complete his Craven Cottage return.
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His Newcastle team-mates are preparing to take on Porto at the Estadio do Dragao as they step up their pre-season preparations.
Mitrovic divided opinion on Tyneside but his exit is likely to mean another forward arrives at St. James’ Park this week.
Salomon Rondon is a name being touted at present, with Dwight Gayle potentially being used as a makeweight in any deal.
Newcastle fans have voiced their anger at owner Mike Ashley recently after a quiet transfer window.
Fabian Schar joined the north east outfit this week, while Yoshinori Muto’s move is expected to be finalised in the coming days.
Fulham’s Mitrovic deal means the west London club have brought in five new names this summer.
Jean-Michael Seri, Maxime Le Marchand, Andre Schurrle and Fabri have all joined the newly-promoted side recently.