The 24-year-old is Erik ten Hag’s first signing of the summer in a deal worth an initial £55m with a possible £5m in add-ons.
“I am hugely ambitious. I know how amazing it feels to win major trophies and what it takes to do it,” said England international Mount.
“I will be giving everything to experience that again at United.”
The Brazil forward, 31, was a free agent after his Liverpool contract expired at the end of last season.
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He is one of several big-name signings by Saudi clubs this summer and joins Senegal goalkeeper Edouard Mendy at Al-Ahli after he left Chelsea last week.
N’Golo Kante and Kalidou Koulibaly have also left Chelsea for Saudi Arabia, with Karim Benzema, Ruben Neves and Marcelo Brozovic among the others to move, while ex-Liverpool midfielder Steven Gerrard has become manager of Al-Ettifaq.
The trend underlines the league’s ambition to be one of the top five in the world and follows a decision in June by Saudi Arabia’s Public Investment Fund (PIF), which owns Newcastle United, to take over four leading clubs in the country.
Firmino left Anfield as a free agent this summer, along with Naby Keita, Alex Oxlade-Chamberlain and James Milner.
Written by: Debby Eselebor
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