A colourful and charismatic fellow, Marchionne was one of the most respected chiefs in the auto industry, having turned around Fiat's waning automotive fortunes.
Selected as CEO of Fiat SpA in June 2004, Marchionne took the Italian manufacturer from the brink of bankruptcy to the New York Stock Exchange, where he rang the bell on October 13, 2014, to mark the debut of Fiat Chrysler Automobiles NV, the London company created when Fiat bought the Detroit carmaker.
This year also saw the Alfa Romeo brand return to F1 as part of Marchionne's vision to get the Italian manufacturer even greater global exposure and drive profits for FCA. He was replaced as FCA CEO over the weekend by Briton Mike Manley. Indeed before last week's crisis, many company executives remained in the dark, four banking people who spoke to Fiat Chrysler executives told Reuters.
Manley said: "This is a very sad and hard time".
In the wider world Marchionne was known for brokering the alliance between Fiat and Chrysler and turning both companies around.
During his 14 years at the helm, Marchionne is credited with revamping Fiat, Italy's premier private enterprise, and bringing it back from the brink of bankruptcy.
In the past four years alone, Fiat Chrysler's stock price almost quadrupled under Marchionne.
FCA has said Manley will work to ensure a "strong and independent" future for the group.
"With Sergio Marchionne Italy loses not only the most brilliant of its managers, but one of the symbolic figures of our country".
Discussing how he changed the culture at Chrysler, Marchionne said in 2013 that he believed if a business has become dysfunctional, "the real problem sits at the top" - so, he added, "we changed all of the senior leadership of Chrysler within a matter of a couple of days".
This description diverges from what Marchionne himself told investors on 1 June during a day-long strategy presentation in Balocco, Italy.
Marchionne had been planning to step down from FCA next year. That included getting GM to pay $2 billion to sever ties with Fiat, key to relaunching the long-struggling Italian carmaker, and the deal with the USA government to take Chrysler without a penny down in exchange for Fiat's small-car technology. Marchionne also revealed earlier this year a new five-year plan for the automaker that calls for several new models and a major push into self-driving and electrification technologies.