The new deal for the 52-year-old was penned just a day before Chelsea begin the defence of their Premier League title at home to Swansea.
“If the club is happy, I am happy. I think this is a normal thing for me to sign a new contract,” said Mourinho.
Mourinho is in his second spell in charge at Stamford Bridge after returning to the west London club in 2013.
The Portuguese coach led Chelsea to the Premier League title and also won the League Cup last season, adding to the two English titles, one FA Cup and two League Cups he lifted in his first spell between 2004 and 2007.
“It is important we have this continuity and I hope we can enjoy more success in the future — for the fans, the players and the club,” Mourinho said.
“I said when I returned here two years ago that I have a special feeling for this club and nothing changed.
“It is the club closest to my heart and I am very happy to know I will be staying here for a long time.”
Following his return to Chelsea, the former Porto, Inter Milan and Real Madrid boss has often spoken of his desire to stay at Chelsea for as long as owner Roman Abramovich wants him.
Mourinho’s original stint at the Bridge ended when he was sacked by Abramovich after a power struggle with the Russian billionaire.
But they repaired their relationship sufficiently for Mourinho to be offered an emotional return two years ago and now he is committed to the club for the long-term.
“We are very happy that Jose has committed four more years to the club,” Chelsea director Marina Granovskaia said.
“Since his arrival two years ago he has carefully developed the playing squad and brought trophies to Stamford Bridge.
“We look forward to the next four years and the continued success of the team.”
Mourinho would dearly love to celebrate his new deal by winning the Champions League and helping Chelsea become the first team since Manchester United in 2008-09 to clinch successive Premier League titles.
But, asked this week what would constitute success this term, Mourinho publicly struck a more cautious note.
“I really don’t know, because it depends not just about you, it’s also about the others,” he said.
“You can have a good season and somebody was better than you. And I don’t consider that failure. I don’t consider that a bad season.
“If somebody was better than you, a little bit better, a couple of points more than you, one goal more than you in a knockout phase, then I don’t think it’s a failure.
“I just think credit to the others. Let’s try to be a good team and let’s try to be here in March, April speaking to you about the possibility of winning something.” – Agence France-Presse