The Spain midfielder, currently with his national side in Brazil, returns to the Premier League after Barcelona accepted an initial £27m offer from Stamford Bridge with £2.5m in add-ons based on appearances.
Arsenal declined to exercise an option to buy back the former captain they sold to Barcelona three years ago but are due a sell-on fee which may end up being £2.5m.
Mourinho has moved quickly to fill the impending void left by Frank Lampard’s departure from Chelsea by giving the 27-year-old a five-year deal. Both men trust the transfer can ensure there is no repeat of last season’s trophy-less campaign in west London.
By the end of his contract Fábregas intends to have completed “unfinished business” in the Premier League and will presumably hope to leave Arsène Wenger believing he made a big mistake in not welcoming him back to the Emirates.
“I do feel I have unfinished business in the Premier League and now is the right time for me to return,” said Fábregas who spent eight years at Arsenal after coming through Barcelona’s youth system.
He hankered for a return to Spain but, despite collecting four winners’ medals in his first season and celebrating the first league title of his career the following year, things did not proceed entirely as he might have hoped at Camp Nou.
Although he scored 35 goals in 129 appearances, Fábregas struggled to hold down a regular place at a club in which Xavi, Andrés Iniesta and Sergio Busquets were regarded as the preferred central midfield combination. Accordingly, he was heavily linked with a move to Manchester United last summer.
Twelve months on, Wenger did not jump to re-engage him. “Yes, everyone knows that Arsenal had the first option to sign me,” Fábregas acknowledged. “They decided not to take this option and therefore it wasn’t meant to be. I wish them well in the future.
“I considered all the other offers very carefully and I firmly believe that Chelsea is the best choice. They match my footballing ambitions with their hunger and desire to win trophies. They have an amazing squad of players and an incredible manager. I am fully committed to this team and I can’t wait to start playing.
“It was extremely important to me that this transfer was completed before the World Cup so that my mind will only be focused on hopefully helping my country try to retain the trophy.”
With Barcelona, who failed to win any silverware last season, hiring Luis Enrique as head coach last month, they have embarked on what is expected to be a transitional phase.
Fábregas was deemed surplus to requirements but remains adamant he harbours no regrets about returning. “Firstly, I would like to thank everyone at FC Barcelona where I enjoyed three wonderful years,” he said. “It was my childhood club and I will always be proud and honoured that I had a chance to play for such a great team.”
Fábregas is likely to be joined by Diego Costa, his Spain team-mate, at Chelsea before the start of the new season. The Atlético Madrid forward has long featured on Mourinho’s shopping list and looks close to being finally captured by the Portuguese, although Chelsea are becoming frustrated by delays in the move after the striker’s successful medical.