Ibrahimovic, 35, joined United from Paris Saint-Germain in July on a one-year contract with an option for an extra year.
"The Zlatan situation is simple," Mourinho told reporters at Old Trafford ahead of United's Europa League meeting with Feyenoord on Thursday.
"He has one plus one year in the club. We are happy with him. We are going to executive the option of a second season. After that he can do what he wants to because he is free."
Ibrahimovic, appearing at the same press conference, said he would be happy to extend his contract if he is still enjoying life at the club.
"I have a contract which says one plus one. I feel good, I feel fresh, in shape," he said.
"If I feel like (I do) now, it will be a second year. I want to be realistic with myself. If I'm here I want to be able to perform, do what I can do and not waste time. As it is now, probably, yes."
Mourinho previously spent a season working with Ibrahimovic at Inter Milan, where the pair won the 2008-09 Serie A title.
"I think for the next one and a half years he is happy, he is committed and loving his career at Man United," Mourinho added.
"This is a great and probably the last big challenge of his wonderful football career. It's perfect for him to be here for 18 months more and then he owns his decision of his future."
Ibrahimovic, who has won 11 league titles in four countries, has scored eight goals in his 17 United appearances to date.
The charismatic Swede went off the boil after scoring five goals in his first five games, but recently ended a six-game Premier League scoring drought with a brace in a 3-1 win at Swansea City.
He missed United's 1-1 draw with Arsenal last Saturday through suspension.
Ibrahimovic scored a record 62 goals in 116 appearances for Sweden before retiring from international football after his country's group-stage exit at Euro 2016.