Terry signed a one-year deal in May and he has showed no signs of letting up in his 17th season at Stamford Bridge, helping Chelsea maintain their three-point lead atop the Premier League.
The 34-year-old, who played his 645th match for Chelsea on Boxing Day, appears set to extend his stellar career with Mourinho revealing a contract extension is imminent.
"There are no doubts that he is going to get another contract," said Mourinho ahead of Sunday's trip to Southampton.
"I know that, he knows that. We in the club, in the board, we all know that and I think, also, you have to feel that.
"Without us confirming anything you have to feel that. He's a very important player in the team so the next contract, sooner or later, will come. It's a formality."
Chelsea have threatened to run away with the title this season, in part to Terry's fine form at the heart of defence.
Terry has turned back the clock in 2014-15, scoring two goals for the league leaders, including the opener against West Ham on Friday.
"He's a happy guy, he's playing well, he likes his team-mates, he likes the manager and he is working well," said Mourinho.
"It's a very happy group and I think John feels very, very well."
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