Spain midfielder Mata was in the last six months of the deal he signed when he joined the club from Chelsea in 2014, but United had the option of keeping him for an extra season and have triggered that agreement.
The 29-year-old is now committed to United until 2019.
Asked if the contract had been extended, United manager Mourinho said: "Yes it was. Last week or something like that."
There was speculation about Mata's future when Mourinho took charge at United in 2016.
Mourinho was manager at Chelsea when Mata was allowed to leave Stamford Bridge four years ago.
But Mourinho said on Tuesday: "Mata is very important. He is another one, big story when I arrived one and a half years ago - 'Mata in trouble, in trouble, in trouble'.
"Now he's getting an extension of one more year. He is an important player for me, an important player for the club, an important player for the other players."