The Portuguese coach travelled to London on Monday to complete the formalities of a second stint in charge of the west London club after his exit from Real Madrid was confirmed, penning a four-year deal.
Mourinho was headhunted by the Blues after guiding Porto to Champions League glory in 2004, and would lead Chelsea to successive Premier League titles in 2005 and 2006, before then leaving after his relationship with owner Abramovich broke down and they parted company, officially by mutual consent at the time.
Since winning the European Cup again with Inter Milan and the Spanish title in his second season in Madrid, the 50-year-old has never made any secret of a love affair with the Chelsea faithful.
Mourinho's relationship with Abramovich has long since warmed and the Portuguese maintains it was an "easy decision" to make.
"I had to prepare myself not to be too emotional on my arrival at the club, but obviously I am very happy," Mourinho told Chelsea TV.
"It was an easy decision. I met the boss, I met the owner and in five minutes after a couple of very short but pragmatic questions, we decided straight away.
"I asked the boss 'do you want me back?' and the boss asked me 'do you want to come back?' and in a couple of minutes, the decision was made."