The 23-year-old had previously seen moves to Brighton & Hove Albion and Anderlecht break down in the closing days of the transfer window, but finally sealed his move away from St James' Park just before the 11pm deadline.
Mitrovic has found first-team football difficult to come by for the Magpies so far this season, making just one start and seven appearances across all competitions since their promotion back to the Premier League.
"I'm thrilled to welcome Aleksandar to the club. He is a talented player who has already amassed an impressive record at the highest levels of the game," Fulham director of football Tony Khan told the club's official website.
"We're looking forward to him joining the Fulham squad and helping us build towards our ultimate goal of promotion this season."
Mitrovic could make his debut for the Cottagers at home to Nottingham Forest on Saturday.