Giannino was one of four racing sons of Count Gaetano Marzotto. He entered his first competitive event shortly after his twentieth birthday, campaigning a Lancia Aprilia in the Giro di Sicilia and finishing second in class. He met Enzo Ferrari in 1948, commissioning what is believed to be the fourth customer car ever produced by Ferrari, a Ferrari 166 Inter. Over the next three years he drove the Aprilia in ten Italian national championship events while building his relationship with Maranello.
The 1950 Mille Miglia became the race that defined Marzotto's legend. He, along with brothers Vittorio, Umberto, and Paolo, all entered the event driving Ferraris, making it a family affair at the grandest scale. Starting last on the road, Giannino drove a Ferrari 195 S with co-driver Marco Crosara to an outright victory. The win was celebrated not only for its sporting merit but for an enduring image: Marzotto crossed the finish line wearing a double-breasted brown suit, a nonchalant elegance that seemed to embody the Italian sporting spirit of the age. Enzo Ferrari later recalled that his technician Luigi Bazzi had deliberately strangled the engine beforehand to protect the young driver, and that Enzo personally oversaw the car's preparation for the race after Giannino confronted him at Maranello.
The following year, Marzotto designed a special car of his own — a low-slung body based on the Ferrari 166 chassis but powered by a Ferrari 212 engine. The car ran well in the 1951 Mille Miglia but retired with rear axle trouble.
In 1951, Marzotto undertook a limited campaign in Formula 2, racing for the family team Scuderia Marzotto as well as in works Ferrari machinery. He won the Grand Prix de Rouen-les-Essarts and visited the podium at the Gran Premio di Roma. He was entered for two Formula One races that year — the non-championship Grand Prix d'Albi and the Italian Grand Prix — but took part in neither; his entry was withdrawn from the former and his car was unavailable for the latter.
The 1953 victory was arguably more dramatic than the first. Originally intending to race an Alfa Romeo 6C 3000 CM, Marzotto returned from a business trip to Lebanon to find the car had been reassigned. Gianni Lancia's team was fully committed. Left with no option, Marzotto approached Ferrari despite a strained relationship between the two parties. He was shown Luigi Villoresi's 340MM, which had won the Giro di Sicilia but had not been touched since that race, having suffered brake failure near the finish. The car was not delivered to him until hours before the start, leaving no time for testing.
During the race, Marzotto realized Ferrari mechanics had failed to change his engine oil at the Siena control because they could not open the bonnet. He made a U-turn, returned to Siena, and his mechanics cut a hole directly over the oil filler cap to top up the oil. Despite two minor accidents, and after Juan Manuel Fangio's leading Alfa Romeo fell back with steering and brake problems, Marzotto pressed on to Brescia for his second victory, again accompanied by Crosara.
The 1954 Mille Miglia, in which he was partnered by Gioia Tortima instead of Crosara in a full works Ferrari, would be his last international race; he withdrew during the event through illness.
Enzo Ferrari held Marzotto in exceptional regard. He described him as "a real sport merchant, a young Varzi in his cold, calculating earnestness," and believed that his Mille Miglia successes showed he could have become a professional champion. Marco Crosara, his co-driver in both victories, died on 19 July 2012, just five days after Marzotto himself.
After retiring from motorsport in 1953, Marzotto joined the family firm, G. Marzotto & F. Spa, rising from director to managing director, then vice-president, and finally president in 1968. He married in 1969 and had three daughters.
Marzotto served as President of the Mille Miglia Club from 1988 to 1990 and again from 2011 until his death. He was Honorary President of the Luigi Bonfanti Automobile Museum, which dedicated a permanent section to his career. The Galleria del Motorismo, Mobilità ed Ingegno Veneto — Giannino Marzotto explores his life in motor racing and is dedicated entirely to him.