The Cottagers have been wrangling with Newcastle over a price for the Serbia striker, who was a key figure in their promotion push after spending the second half of last season on loan at Craven Cottage.
When Benitez was asked if he will get the full amount of the Mitrovic fee to spend this summer, he said: "I don't know the numbers".
The 23-year-old told the club's official website: "I'm very glad and happy and proud to finally be here and officially become a Fulham player".
"I have love for the fans".
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"The 23-year-old striker proved to be the ideal fit for Fulham, scoring 12 times in just 17 starts to help the team reach Wembley".
"I'm delighted to announce that Aleksandar Mitrovic is back where he belongs at Fulham Football Club!"
A technically gifted side, Fulham will benefit from the physical presence Mitrovic provides in the coming campaign.
The 26-year-old has 20 goals and ten assists in the Bundesliga for Mainz (eight and four respectively last season), and was also a member of Japan's World Cup squad in Russian Federation, featuring in the group match against Poland.
Speaking after putting pen to paper on a five-year deal, Mitrovic also expressed his delight at returning to the club.