From the window of his new office, he looks out at the Forum and the Colosseum, and the great gash of highway that runs between them, the Via dei Fori Imperiali. Rammed through Rome's ancient core in 1932 by Benito Mussolini, the road had become a snarling mass of honking Fiats, darting Vespas and lumbering buses, all spewing exhaust onto one of the world's premier archaeological sites.
So, just seven weeks into his term, Marino, 58, ordered the road closed to nonessential traffic on weekdays and banned motorized vehicles on weekends.
Romans, who have the highest car-ownership rate of any city in Europe, were initially shocked to lose access to a major thoroughfare. But Marino assured them they would get used to it.
"Philadelphia," he explained, "taught me how to go around a city without a car."
Even though Marino spent his formative years in cities whose narrow streets encourage walking - Genoa as a child, Rome for medical school - he said it wasn't until he moved to Philadelphia in 2003 that he lived in a city without owning a car.
At the urging of his daughter, the family sold its car. To get around, Marino picked up a used Schwinn at Via Bicycle on Ninth Street. He soon became a distinctive sight in Philadelphia, whizzing around on his red bike in his green hospital scrubs. When his family needed to run errands, Marino said, he simply rented one from the nearest car-share pod.
"It was a nice life," he said.
But the pull of home was strong. Italy's economy was stagnating, and its political culture had become moribund under former leader Silvio Berlusconi. Although Marino was a top liver-transplant surgeon, he felt he could accomplish more through politics.
In 2006, he ran for a seat in the Italian Senate and won. Returning to Rome, Marino decided to live just as he had done in Philadelphia.
At first, his Roman friends were incredulous, Marino said. But like London's bike-riding mayor, Boris Johnson, he wanted to use his office to crusade for environmentally friendly alternatives to car ownership. When he became mayor, he bought a new mountain bike. Now he is now accompanied on his travels by a pair of bike-riding polizia.
During the campaign, Marino vowed to make the car-choked Italian capital a more livable place, starting with the closure of the Via dei Fori Imperiali to cars.
Unlike Parisians, who have taken to their Velib bike-share system with a vengeance, or Londoners, who have pedestrianized part of Trafalgar Square, he felt Romans had been slow to adopt planning ideas that favor walkers and bicyclists - even though they practically invented the gracious pedestrian plaza. Though Rome's cobbled lanes are perfect for strolling, Italians, who even own more motor vehicles per capita than Americans, are obsessed with cars and speed.
The Via dei Fori Imperiali is a particularly important road. It cuts through the center of the city, much like Philadelphia's Broad Street, but it has done irreparable harm to the Forum. Because cars pass just feet from the Colosseum, the exhaust has turned its white marble walls a dull gray.
"I've always felt that a 2,000-year-old monument should not be used as a roundabout for traffic," Marino said.
For years, the European Union begged the Italian government to protect the Forum, considered a treasure of the world's heritage. Not much happened.
After getting past their initial shock over the closure of Via dei Fori Imperiali, Romans have flocked to the car-free boulevard. On weekends, it is packed with a parade of stylish strollers and spandex-clad bicyclists making the loop between the Colosseum and the Piazza Venezia.
As a scientist, Marino said, he sees the road's pedestrianization as the equivalent of a clinical trial. Once Romans have grown more comfortable with the new traffic patterns, Marino hopes to ban vehicles permanently.
In late October, he reopened the ancient Via Alessandrina to pedestrians, allowing access to Trajan's Market. He plans another car-free zone, on the Via del Corso, a major shopping street. He wants to ban cars on several streets near the Spanish Steps.
By next summer, Marino hopes to have a bike-share program in operation. "Just 2,000 bikes, not as many as Paris, but a start," he said. He also plans to expand the city's meager network of bike lanes - now 150 kilometers, about half the size of Philadelphia's.
Cataldo Doria, who directs Jefferson's division of transplantation, and who was also born in Italy, said he was not surprised Marino had accomplished so much. He said Marino, who defeated an entrenched, center-right candidate associated with Berlusconi, represented a new kind of politician for Italy.
"He's not one of those politicians always going around in a limousine with a blue light," Doria said.
Still, Romans have not wholeheartedly embraced Marino's changes. After the Via dei Fori Imperiali was closed, Berlusconi's party plastered the area with posters declaring, "No to the pedestrianization."
Ale Felici, who lives nearby in the gentrifying Monti neighborhood, said she loved being able to stroll and bike around the Colosseum without being terrorized by traffic. But she concedes Marino still needs to change the Roman mind-set.
"This is Italy, after all," she said. "We are not one of those orderly, Northern European countries."