Backus, who is the nephew of boxing legend and former world champion Carmen Basilio, started his career by winning only 7 of his first 19 bouts. From February 27 of 1963 to January 19 of the same year, he had a three fight losing streak. In 1964, things looked promising when he won four bouts in a row, but then he suffered another three fight losing streak. At this point of his career, he had a pretty average record of 7 wins, 7 losses and 3 draws and he retired from boxing. To make things worse, he had lost his last fight, against Rudy Richardson, on his 22nd. birthday.
Backus had to resume his boxing career in 1966, but probably never in his wildest dreams could he have envisioned what happened next: In his comeback bout, he knocked out Tod Purtell in the first round, and that marked his embarkment on a seven fight winning streak that included avenging an earlier loss to Dick French. That streak was broken by a defeat at the hands of Percy Pugh in New Orleans, but soon he started on another winning streak, eight in a row, including two over Pugh. Then, Pugh broke his winning streak once again, beating him over 15 rounds, once again at New Orleans. After splitting two fights with Jerry Pellegrini and drawing in four rounds with Ricky Ortiz, Backus embarked on another winning streak, including an eight round knockout over Ortiz in a rematch.
After that last streak, Backus was ranked among the top ten Welterweight challengers of the world at number 10. The year was 1970 and world champion Jose Mantequilla Napoles had earned the right to make an optional defense (meaning he could pick any challenger among the top ten to defend against), and his management, figuring they'd have an easy fight ahead of them, picked Backus as the challenger for this optional title bout. But Backus landed a punch that opened a cut over Napoles' eye in round one, causing Napoles to bleed profusely, and the fight was stopped in round four. Since the cut had been ruled to be caused by a punch, Backus was declared winner by a technical knockout, and he had realized a dream that many fans never thought he would: he had become the world's Welterweight champion. His uncle Basilio, watching from ringside, said: Billy winning the world title is the best thing ever to happen in my life, even better than me winning the world title.
Billy won two non title fights, including one over Robert Gallois in Paris, and then had a rematch with Napoles in Los Angeles, but this time Napoles returned the favor, knocking Backus out in the fourth round and regaining his title.
He finished 1971 with a ten round decision win over Jose Gabino, and went 3-2 in 1972, losing two fifteen round decisions to world title challenger Hedgemon Lewis. He lost 3 of 4 bouts in 1973, and went on an European tour in 1974, winning 2 and losing 2 in Paris and Berlin. In 1975, he went to Australia and lost by a knockout in five to Rocky Mattioli, and then he came back to the States, where he beat Marc Gervais by a knockout in ten. This marked the start of another 9 fight winning streak, which made the WBA make him their number one challenger. After drawing in twelve rounds with Everaldo Acosta Acevedo, he had a chance at regaining the world welterweight title, when faced against world champion Pipino Cuevas of Mexico, once again in Los Angeles. After losing by a knockout in the first round, Backus then announced his retirement from boxing for good.
Although he isn't a member of the International Boxing Hall Of Fame (unlike his uncle Basilio, who is), in 1990, Ed Brohpy and some Canastota businessmen came up with the idea to build the hall of fame in Canastota, to honor Canastota's two native world champions: Basilio and Backus.
Backus had a final record of 48 wins, 20 losses and 5 draws, with 22 wins by knockout