Marzotto was one of four sons of Count Gaetano Marzotto, a prominent Italian industrialist. All four brothers — Giannino, Vittorio, Umberto, and Paolo — were drawn to motor racing. Shortly after his twentieth birthday Giannino entered his father's Lancia Aprilia in the Giro di Sicilia, finishing second in class. His introduction to Ferrari came in 1948 when he met Enzo Ferrari with the task of building a 2-litre grand touring coupe; the resulting car, a Ferrari 166 Inter, is believed to have been one of the earliest customer Ferraris ever built.
In 1950 all four Marzotto brothers entered the Mille Miglia driving Ferraris. Giannino, starting last, drove a Ferrari 195 S to overall victory with co-driver Marco Crosara. The win became legendary not only for the result but for the image it produced: Marzotto took the wheel wearing a double-breasted brown suit, an image that seemed to embody the Italian amateur-gentleman spirit of the race.
The victory almost did not happen. During pre-race testing Marzotto found the Ferrari unsatisfactory compared to his previous car and returned to Maranello to confront Enzo Ferrari. It emerged that technician Luigi Bazzi had deliberately restricted the engine to protect the young driver. An embarrassed Ferrari promised the car would be personally prepared for the race.
In 1951 Marzotto undertook a limited Formula 2 campaign for the family team, Scuderia Marzotto, and also drove for the works Ferrari outfit. 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Prix d'Albi and the 1951 Italian Grand Prix for Scuderia Ferrari — but took no part in either: the first entry was withdrawn, and the car assigned for Monza was unavailable.
Marzotto's second Mille Miglia victory, in 1953, was one of improvisation and perseverance. Having planned to race an Alfa Romeo 6C 3000 CM, he returned from a business trip to Lebanon to find the car had been reassigned. A call to Gianni Lancia found that team fully committed. He was ultimately forced back to Ferrari despite their strained relationship, receiving Luigi Villoresi's untouched 340MM — which had suffered brake trouble at the end of the Giro di Sicilia — only a few hours before the race.
During the event, approaching Siena, Marzotto realised the Ferrari mechanics had been unable to top up the engine oil at the last control point because they could not open the bonnet. He turned back to Siena, where mechanics cut a hole in the bonnet directly over the oil filler cap to complete the service. Meanwhile the leading Alfa Romeo of Juan Manuel Fangio was falling back with steering and brake trouble, and Marzotto went on to win his second Mille Miglia alongside Crosara.
The following year's race proved to be his last international start. Partnered by a new co-driver in place of Crosara, he retired during the event through illness.
Enzo Ferrari, who knew the Marzotto brothers as important customers, wrote of Giannino: "He was a real sport merchant, a young Varzi in his cold, calculating earnestness. The successes in the Mille Miglias were a sure sign of his dedication and enough to show that he would have made an excellent professional driver, perhaps even a champion."
After retiring from racing Marzotto joined the family textile firm, G. Marzotto and F. Spa, as a director in 1953, was promoted to managing director in 1956, became vice-president and managing director in 1958, and ultimately served as company president from 1968. He married in 1969 and had three daughters.
Marzotto served as president of the Mille Miglia Club between 1988 and 1990, and again from 2011 until his death in 2012. He was also Honorary President of the Luigi Bonfanti Automobile Museum, which dedicated a permanent section to his career. A gallery named Galleria del Motorismo, Mobilita ed Ingegno Veneto — Giannino Marzotto tells the story of his life in motor racing and his industrial heritage. His co-driver Marco Crosara died on 19 July 2012, just five days after Marzotto.