"Zlatan's situation is simple: he has a one-plus-one contract with the club and we will execute the option of a second season," Mourinho told reporters.
The former Sweden captain, who was suspended for Saturday's 1-1 Premier League draw at home to Arsenal, ended a six-game goal drought in a win over Swansea City earlier this month.
"He's happy, committed and loving his life as a football player at United," Mourinho added.
"This is probably the last big challenge of his wonderful career, so it is perfect for him to be here for the next 18 months and then he owns the decision of his future."
Ibrahimovic said the extension is "automatic after a while", but the criteria for it to be triggered has yet to be fulfilled.
"We didn't even have any discussions. I feel good, I feel fresh, I feel like now I'll probably take a second year," he said.
"I want to be honest with myself and I don't want to waste time. The second year goes automatic after a while, there's no word from me and no word from them. If it continues like this then it is a yes."
Ibrahimovic also leapt to the defence of Paul Pogba, who returned to United from Juventus for a world record fee of 89 million pounds in August, saying the France international needs more time to find his feet at the club.
Pogba has yet to light up the Premier League, scoring just twice in 11 games, as the powerful midfielder struggles to cope with the pace and intensity of English football.
"There is big pressure on him because people expect magic in a short time instead of being realistic," said Ibrahimovic.
United, who are third in their Europa League group, host second-placed Feyenoord on Thursday before facing West Ham United in the league three days later.
(Reporting by Shravanth Vijayakumar in Bengaluru; editing by Ken Ferris)
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