Mitrovic was expected to return to former club Anderlecht in the latter stages of the transfer window, but the move broke down on Wednesday afternoon.
"It was a really long and stressful day but I am here and I want to be here," Mitrovic told Fulham's official website.
"There were a lot of clubs [interested] but I chose Fulham, first of all because of the coach and second of all because of their style of football.
"Fulham have a young and good team and they have scored a lot of goals this season and last season as well. For me, Fulham played the best football in the Championship last season and they have continued to play like that this season as well."
Mitrovic has scored twice in seven appearances for Newcastle this season.