Tilelli was born in Brooklyn, but lived most of his life in Philadelphia, where as a young man he joined many other Italian-Americans in the city in taking up boxing. He turned professional in 1948, not long after his 18th birthday.
As a pro, he quickly racked up an 18-0-1 record in his first 19 fights while facing less-than-stellar opposition. He fought just three men who had previously won a fight during that time. It caught up with Tilelli on January 16, 1950, when he was handed his first defeat by Joe DiMartino, a journeyman with a 6-10 record.
After that embarrassment, he began to face better opposition and by 1951, was beating some of the better middleweight boxers on the Philadelphia scene. He continued to do so for years afterward, but was blocked from receiving a shot at the world championship by the underworld figures who controlled the sport at that time.
It wasn't until 1960 that Tilelli, now known as Giardello, received any kind of championship opportunity. On April 20, he faced Gene Fullmer for the National Boxing Association version of the world middleweight title. He missed out on the title when he and Fullmer fought to a draw over 15 rounds.
Giardello lost four of his next six fights, but then came back strong with an 8-1-1 record in his next 10, all of which were over some of the biggest names in the division at that time. One of his wins, a 10-round decision over Henry Hank on January 30, 1962, was chosen as Ring Magazine's fight of the year. Then, on June 24, 1963, Giardello upset boxing legend Sugar Ray Robinson, and at the age of 33, was finally named as the No. 1 challenger for the world middleweight title.
He didn't waste the opportunity. On December 7, Giardello faced Dick Tiger in Atlantic City for the title and won, taking the world championship by decision in 15 rounds and drawing with Tiger in two others.
He reigned as world champion for nearly two years, winning four fights during that time. The most notable was a December 14, 1964 title defense against Rubin Carter, which many today believe he won on an unjust decision because of the fight's portrayal in the 2000 movie The Hurricane. However, most boxing experts of the time believed he deserved the victory.
Giardello gave Tiger a rematch on October 21, 1965 and this time, the Nigerian decisioned Giardello over 15 rounds to regain the belt. Giardello fought just four more times over the next two years before retiring.
After retirement, he went into private business and went back to his real name, keeping a generally low profile until the movie about Carter's life came out. Angry that his win over Carter was portrayed as a racist "fix", he sued the movie's producers and won a settlement.
He was named to the International Boxing Hall of Fame in 1993. His career record was 101 wins, 25 losses and 7 draws, but even more impressively, he was 5-3-1 against other boxers in the Hall of Fame, including a 2-2 mark against Tiger.