Pep started boxing professionally on July 10 of 1940, beating James McGovern by a decision in four rounds in Hartford. Like many boxers of the first half of the 20th century, Pep concentrated his early fighting career on boxing in New England, and he split his first 25 contests between Connecticut and Massachusetts. He was undefeated during that span, and for fight number 26, he finally headed 'west', beating Eddie Flores by a knockout in the first at Thompsonville, Michigan. A couple of fights later, he travelled further west and made his California debut, beating Billy Spencer by a decision in four at Los Angeles
By the time Pep stepped up his quality of opposition when he met world title challenger Joey Archibald in 1942, he was already 41-0. He beat Archibald by a decision in ten, and in his next bout, he challenged Abe Denner for the New England area Featherweight title. He won the fight by a decision in 12, and his status among the world's top Featherweights kept on rising.
He won ten more bouts to reach 52-0, including a rematch win over Archibald, before he was given his first world championship try in October of that year. He became the world's Featherweight champion by outpointing the defending world champ Chalky Wright, over the 15 round distance. He fought twice more to finish the year, winning both by knockout.
Pep began 1943 by winning six bouts in a row to find himself with a record of 61-0. But in his seventh bout of 1943, he suffered his first defeat, at the hands of Sammy Angott, another world champion boxer. Angott beat Pep over the ten round distance, by decision. Ten days later, Pep was back in the ring, beating Bobby McIntyre by a decision. He closed 1943 winning five fights in a row, including two over future world champion Sal Bartolo and one over Jackie Wilson (boxer). The second win over Bartolo was in a defense of the world title.
1944 was a very good year for Pep. He won all 16 of his bouts that year, including wins over world Bantamweight champions Willie Joyce and Manuel Ramos, fringe contender Jackie Lemus and Wright, who was beaten two more times, one time with Pep's crown on the line. He also made his first fight abroad, because the fight with Lemus was held in Canada.
He had eight fights in 1945, winning seven and drawing one. He beat former world champion Phil Terranova to retain the title, and had a ten round draw with Jimmy McAllister.
In 1946, Pep had 18 fights, and won all of them, including a 12 round knockout of Bartolo and a three round knockout of Wright. He had a 6 fight knockout win streak during a span that year.
1948 was a year that would become important in Pep's life: He won 15 bouts before going into what would be the first fight of his four fight series with Sandy Saddler. He retained the title by beating Humberto Sierra by a knockout in 10, and he beat former world champion Paddy DeMarco, also in ten, but by decision. Then, on October 29, he lost the world Featherweight title to Saddler by a knockout in four.
After two wins, he and Saddler met in 1949, on their rivalty's second installment, Pep recovered the worlds featherweight championship by beatin Saddler by a 15 round decision, and then he engaged in a series of exhibition and ten round bouts before defending the crown against Eddie Campo, winning by a knockou in the seventh. He finished that year beating fringe contender Harold Dade by a decision in ten at St. Louis.
In 1950, he won nine fights before meeting Saddler for a third time. Those nine bouts included defenses against Charlie Riley, knocked out in five, and Australias Ray Famechon, beaten by decision in 15. Then came the third fight with Saddler, and Pep once again lost his world Featherweight championship to Saddler, being knocked out in the eighth round.
1951 brought over a hint of controversy to Pep's life. He won eight bouts in a row to start the year, but his ninth bout, the last chapter of the rivalty with Saddler, was his most important bout that year and the one that he lost. He was knocked out in nine by Saddler, and some fans and historians have suggested that he took a dive in this bout. Whether that's true or not remains a mystery, but most boxing experts have agreed that the Saddler-Pep rivalty was one of boxing's most savage and foul infested rivalties in history.
In 1952, Pep had 12 fights, winning 11. He was knocked out in six by Tommy Collins but also held two wins over Billy Lima that year.
Pep won all 11 fights in 1953, and entered 1954 on a 17 fight winning streak. After beating David Seabrooke by a decision, he met fringe contender Lulu Perez. If the fourth fight with Saddler brought a little of controversy to Pep's life, this fight would bring even more. Pep lost by a knockout in two rounds and the rumors that he took a dive in this fight were even more persistent than on the case of the fourth Saddler bout. Reporters from all over the world would write about this fight, commenting about a possible dive by Pep. Pep ended up winning three more bouts before the end of the year.
Pep went on boxing for 5 more years, retiring in 1960, and then he came back in 1964 and boxed for two more years. During that last period of his boxing career, he won 43 bouts and lost only 5, but his only opponent of note during that time was Hogan Kid Bassey, a future world Featherweight champion who knocked Pep out in nine rounds. Pep boxed in Venezuela, losing to Sonny Leon by a decision in ten, and in his last fight, in 1966, he lost to Calvin Woodland by a decison in six.
After retiring, he and Saddler involved in a series of exhibition bouts, and in 1980, Pep sued Sports Illustrated for running a story suggesting that he threw his fight with Perez. Pep won the lawsuit, and Sports Illustrated had to pay him a large amount of money, on the base that the article was detrimental to Pep's personality and not based on proven facts. In 1990, Pep was inducted into the International Boxing Hall Of Fame as a member of the Hall's original class. Pep, who enjoys giving out autographs to his fans, works there as a volunteer alongside fellow former world champion boxer and friend Carmen Basilio.
Pep had a record of 230 wins, 11 losses and 1 draw, with 65 wins by knockout, making him a member of Ring Magazine's exclusive group of fighters with 50 or more career knockout wins.