The CEO of FCA since 2004, Marchionne has long said he anticipated stepping down in 2019.
Marchionne, widely credited with rescuing both Fiat and Chrysler from the brink of bankruptcy, had focused on fixing FCA's finances first, notably erasing all debt.
Another Italian newspaper, La Repubblica, said on Sunday that the 66-year-old had been in a coma throughout the weekend and was on a ventilator. In that period the FCA Group has seen a quadrupling of Jeep sales.
Marchionne resurrected one of Italy's biggest corporate names and revitalized Chrysler, succeeding where the US company's two previous owners - Mercedes parent Daimler and private equity group Carberus - both failed.
Mike Manley talks about the 2017 Jeep Compass at Los Angeles Auto Show in 2016 as head of the Jeep brand.
Marchionne, who was also Chairman and CEO of FCA, had been running Ferrari since taking over from Luca de Montezemolo in 2014.
Ferrari announced some midterm targets earlier this year - pledging to double core earnings and churn out hybrids and an SUV - but a detailed strategy was due in September.
In addition to being FCA chief executive, Marchionne was also CEO and chairman of luxury sports vehicle brand Ferrari and chairman of truck and tractor maker CNH Industrial, which were spun off from FCA in recent years.
His previous roles in the company have included being the head of its Asia Pacific operations, and product chief for all regions except North America.
She called Manley a "worthy replacement at FCA, but it's a huge job to not only fill Sergio's shoes, but to run many brands that are facing capricious fortunes in a variety of markets".
The Italian Canadian manager later demonstrated his agility by refocusing USA production on trucks and SUVS and away from passenger cars to meet market demand, a process that got underway in 2016.
Marchionne's tenure also has been controversial at times. And Sergio has always made a difference, wherever his work took him and in the lives of so very many people.