The England worldwide centre-half joined the Old Trafford outfit as a youngster from Fulham and has been a important player for the club over the years.
The Manchester outfit also have the likes of Phil Jones, Victor Lindelof and Eric Bailly to select from in the position, but Smalling commands respect and is a player Mourinho picks when he is available.
He has gone on take in over 300 appearances for the club and hopes there will be many more to come.
Smalling tweeted that he is delighted to get a new contract.
'It is a real honour to play for Manchester United and we are all now concentrating on the busy schedule of games throughout the festive period'.
The former Fulham man has been a mainstay this season and arguably Jose Mourinho's most reliable centre-back so far as his teammates fitness and form continues to crumble.
The England global has now extended his deal until 2022.
"Chris has been with the club for a number of years and is now one of the senior players within our squad", Mourinho told United's official website.
Smalling's previous deal ran out at the end of the season.
He has since then won two EPL titles, one Europa League cup among other domestic trophies at Old Trafford.