On January 1st, Marchionne sealed the deal to buy the 41.5 percent of the number three US automaker that Fiat did not already hold, after months of tough negotiations with the seller, the union's retirement health fund.
He made no effort to hide his happiness over the deal in Detroit, calling it "the high point of 2014 for me."
"It had to get done; it's like going to the dentist," he said.
The deal marked the 62-year-old's first decade as the chief executive of the Italian group, and four and a half years as CEO of Chrysler. Fiat took a stake in the American automaker in 2009 as it emerged from bankruptcy reorganization.
Since then, Chrysler has rebuilt its business, selling 498,000 cars last year and generating profits that have since become crucial for Fiat, struggling in its own depressed core markets.
Marchionne's efforts have been labeled "la rivoluzione di Marchionne" - the Marchionne Revolution - since he first rescued Fiat from implosion in the 2000s.
"He has done an extraordinary job. He took a company most people thought was going to die and he turned it around very quickly... and now he's pulled off this purchase," said IHS industry analyst Tom Libby.
Karl Brauer, of Kelley Blue Book of auto industry specialists, said Marchionne smartly took advantage of Chrysler's impending bankruptcy to lay down the framework for a new global automaker.
"And by executing on the financial and product challenges involved in such a merger, he's been able to turn that vision into a reality, even in the face of dwindling sales in Fiat's home market."
Marchionne is hardly the typical auto industry chieftain, with years of background in a carmaker's engineering and sales divisions.
Born in Italy, his family moved to Canada when he was 14, where he went to university and became an accountant.
With his rimless glasses, greying hair and ever-present dark pullover, and his regular trips outside conference buildings to feed a heavy smoking habit,
Marchionne still looks like an accountant although he has now spent more than two decades running industrial firms.
But, as with his reputation as a tough negotiator and powerful poker player, he misses little and keeps sharp control over all aspects of the Fiat group, from design to advertising to finances.
He makes a specialty out of hunting down and eliminating excess costs.
"Waste is unethical," he likes to say.
Marchionne himself says he sleeps little and lives in the air, jetting from one Fiat or Chrysler operation to another.
The job demands it. Even if Chrysler is now humming, Fiat itself continues to bleed losses, a hefty challenge for the binational, bilingual CEO.
The father of two is in an odd partnership at Fiat with chairman John Elkann, the always formally dressed, discreet representative of the Agnelli family that controls the company.
Elkann knows he needs Marchionne, who is 24 years his senior.
"We have a deal for at least three years," he told reporters in Detroit, assuring them Marchionne is not going anywhere.
"We built this together."