At the age of 17, while still in school, he signed with the Oaks for a starting salary of $150 a month. He was immediately optioned to the Oaks' Arizona-Texas League team in Phoenix, where he hit .304. After being traded to Bisbee, he came back to Oakland the following year, 1932, when the Arizona-Texas League folded. In 1932 he hit .281 in 42 games with the Oaks.

Raimondi came close to the majors after the 1935 season, when the Yankees exercised their option on his contract and sold him to the Reds. Because of a shoulder injury in Spring training, however, he was forced to sit out the 1936 season. He recovered and returned to Oakland in 1937. Oakland manager Bill Meyer said that, while Raimondi wasn't a great hitter, "he's so far ahead of the other catchers a manager would be silly to have anybody else catch for him." When Meyer moved to Kansas City the following year, he wanted to take Raimondi with him and promised he would move up to the New York Yankees in two years. Bill preferred to stay at home in Oakland and turned down the offer.

Although 1942 was the only season he led the league in fielding, Billy Raimondi was regarded by many as the best catcher in the PCL. His hustling style of play, his dependability, ability to handle pitchers and affable personality made him a PCL star and a crowd favorite around the league.

In 1937, Billy Raimondi did something that few, if any, other players have had the chance to do. In a game on April 8, Seattle led the Oaks 9 to 4 in the last of the eighth, when the game was called on account of rain. The Oaks protested because the umpire only waited 5 minutes, rather than the required 30 minutes before calling the came. Their protest was upheld and the game was resumed 97 days later, the next times the two teams met in Oakland on July 14. Five of the players who were in the lineup when the game started were no longer on the Oaks' roster. Therefore, Billy Raimondi and pitcher Ken Douglas were allowed to return to the lineup to substitute for two players no longer on the team, even though they had been previously replaced in the same game.

Billy's stint as manager of the Oaks came when manager Dolph Camilli left suddenly in mid-season. The team was fourth at 35 and 36 when he took over, and finished fifth. Under Billy they were 54 and 57. Many hoped he would keep the job, but the next season the Oaks hired Casey Stengel.

Raimondi was one of Casey's "Nine Old Men," the Oakland Oaks of 1948, winning the Pacific Coast League pennant in 1948. It was Oakland's first pennant in 21 years. The following year, Stengel went to New York to manage the Yankees. Under Oakland's new manager, Charlie Dressen, Billy was traded to Sacramento.

Billy stayed with the Solons only through the 1950 season, when he was traded to Los Angeles. He played in a total of 133 games in 1949 and 110 games in 1950, hit .266 and .242 -- not bad for a 36 year old veteran catcher. He retired from baseball in 1953, after having played in 21 seasons over his career in the Pacific Coast League, longer than any other field player in the history of the League. As a regular catcher in his first 17 seasons, he caught an average of 131 games a year--158 in 1934. He was elected to the PCL All Star team in 16 of his 21 seasons in the league.