He was replaced four days ago when his health worsened following complications from surgery on his right shoulder.
Mr Marchionne, who was also Ferrari’s chairman, had led the combined company for more than a decade and planned to step down next year.
Mr Marchionne has been succeeded by Briton Mike Manley, head of the Italian-American firm’s Jeep division.
“Unfortunately, what we feared has come to pass. Sergio Marchionne, man and friend, is gone,” said group chairman John Elkann, a member of the Agnelli family that controls the company.
Mr Elkann praised Mr Marchionne’s “values of humanity, responsibility and open-mindedness”.
He added: “My family and I will always be grateful for what he has done.”
Politicians and car industry figures also paid tribute to Mr Marchionne.
Italian President Sergio Mattarella said: “Marchionne wrote an important page in the history of Italian industry. As leader of Fiat, he went through years of very deep and radical transformation of markets, production systems, financial strategies and trade union relations.”
Former prime minister Silvio Berlusconi said: “With Sergio Marchionne, Italy loses not only the most brilliant of its managers, but one of the symbolic figures of our country. He represented the best of Italy.”
Ford’s executive chairman Bill Ford and Renault-Nissan-Mitsubishi chief executive Carlos Ghosn both expressed their sympathies.
Mr Marchionne was seen as the driving force behind Fiat-Chrysler’s growth.
Famous for his signature black jumpers, sharp tongue and workaholic lifestyle, he is credited with saving both Fiat and Chrysler from bankruptcy.
An accountant and lawyer by training, Mr Marchionne joined Fiat in 2004 after the death of its patriarch Gianni Agnelli.
In 2009, he presided over Fiat’s merger with Chrysler, which was then the number three carmaker in the US.
He carried out two successful spin-offs from the merged group – first its tractor-making arm in 2011, which became CNH Industrial, then Ferrari, which was floated in 2016.
By this year, Mr Marchionne had managed to wipe out the group’s near-$13bn (£10bn) debt pile, something he described as a “healing” process for two companies that for so long were associated with failure.
Mr Marchionne was also a big figure in the world of Formula One, and after taking charge at Ferrari in 2014 oversaw a major restructuring of the racing team.
He formed a powerful alliance with Mercedes, fighting major changes to the sport being planned by new owners Liberty Media for 2021.
Mr Marchionne more than once threatened to pull Ferrari out of F1 after 2020 if he was unhappy with the changes.
Chase Carey, the chief executive of F1, described Mr Marchionne as “a great leader”, both for the sport and in the business world generally.
“He led with great passion, energy and insight, and inspired all around him. His contributions to Formula One are immeasurable.
“He was also a true friend to all of us and he will be deeply missed.”
— Sebastian Vettel #5 (@sebvettelnews) July 25, 2018
We at General Motors offer our condolences to Sergio Marchionne’s family and friends. Sergio created a remarkable legacy in the automotive industry. Our thoughts are also with our industry colleagues at Fiat Chrysler as they deal with this sudden loss.
— Mary Barra (@mtbarra) July 25, 2018
RIP Mr Marchionne. You revived an ailing Ferrari
— PrancingDevil (@prancingdevil46) July 25, 2018
We mourn the passing of auto industry legend Sergio Marchionne. He helped multiple companies (Fiat Chrysler, Ferrari, CNH Industrial) tap the public markets, creating thousands of jobs across the globe. Here he is pictured in 2015, the day he took Ferrari public. #RIPSergio pic.twitter.com/r5KA2QcsH0
— NYSE (@NYSE) July 25, 2018
Marchionne was hired to fix Fiat in 2004 when the Italian carmaker was worth $7.5bn. Today he died, with Fiat, Chrysler and Ferrari worth roughly 10 times that. He was the smartest man I ever interviewed, and the car industry will never replace him
— Henry Foy (@HenryJFoy) July 25, 2018
I convey my condolences to the family of Sergio Marchionne and to the entire FCA extended family..
I met Sergio only once but it was clear in an instant that he was a giant among men. @fcagroup
— anand mahindra (@anandmahindra) July 25, 2018
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