Slimani joined on loan until the end of the season, while Mitrovic moved to Fulham after a proposed switch to Anderlecht, his former club, broke down at the 11th hour.
Newcastle, held to a 1-1 draw by Burnley in the Premier League at St James’s Park last night, also signed Sparta Prague goalkeeper Martin Dubravka on a half-season loan.
The capture of Slimani was the most significant.
Rafa Benitez was desperate to sign a striker in last month’s transfer window.
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Slimani – who joined Leicester from Sporting Lisbon in a £28million deal in the summer of 2016 – spoke about his move.
“It’s a great thing for me and for Newcastle,” said Slimani.
“I’m happy to be here. Hopefully, I can come here and be able to give them a hand.
“Knowing the power of the fans and crowd here and how great they are, and also speaking to the manager, it gave me the confidence that this was the right choice.”
United manager Benitez said: “Slimani’s a player with experience in the Premier league, and obviously we were looking for that.
“He is a player who can fight with defenders, challenge and score goals.
“Hopefully, he can give us some experience and some different characteristics to the strikers that we have at the moment.”
Dubravka also reacted to his move to St James’s Park.
The 29-year-old said: “I feel amazing. This is a big step in my career and also in my life.
“You can feel the energy of the people around the club. I will try to work hard, and show that I deserve to be here.”
Benitez said: “Dubravka is a Slovakian international goalkeeper, and he’s someone with some experience.
“He will need to settle down quickly as he is a player coming from abroad, but h’is very agile and he can play with his feet.
“He will give us some competition in goal, and that is what we are looking for.”
Mitrovic, meanwhile, flew to Brussels after Slimani arrived on Tyneside, however, Anderlecht pulled out of the move after failing to offload a player.
And Fulham officials travelled to Belgium to sign Mitrovic, who then underwent a medical in the country before travelling back to England to complete his move to Craven Cottage.
There were also a number of other loan departures.
Midfielder Henri Saivet joined Turkish club Sivasspor, while winger Rolando Aarons linked up with Hellas Verona, managed by Benitez’s former assistant Fabio Pecchia.
Defender Jamie Sterry signed for Crewe Alexandra, while goalkeeper Freddie Woodman – who was on the bench against Burnley – joined Aberdeen.
Midfielder Jack Colback – who has been training at United’s Academy since last summer – signed for Nottingham Forest, also on loan.