Mourinho has penned a two-year extension that brings to an end speculation he could leave the top flight giants.
The Portuguese agreed the new deal last week, as reported in Starsport, but United wanted to wait until Alexis Sanchez had been signed before announcing the news.
Mourinho replaced Louis van Gaal at Old Trafford in 2016 and won two major trophies in his first season, as well as the Community Shield.
He said: “I am really honoured and proud to be Manchester United manager.
“I would like to say a big thank you to the owners and to Mr Woodward for the recognition of my hard work and dedication. I am delighted they feel and trust that I am the right manager for this great club for the foreseeable future.