Born in Los Angeles, California, he joined the U.S. Navy aged 17, and was one of the first divers on the USS Oklahoma at Pearl Harbor.
[2] He won a Grammy in 1964 for Best Engineered Recording - Special Or Novel Effects, for his work on The Chipmunks Sing the Beatles Hits.
[3] From November 1964 until August 1966 he was often the chief engineer for the Rolling Stones, primarily on the dates when they recorded at the RCA studios in Los Angeles, working on several albums and singles with the band during that period.
Hassinger discovered, signed and managed the Electric Prunes, owning the rights to the band's name and engineering all of their recordings from their second single and first hit until their breakup in the early 1970s.
Other musicians with whom he worked included Jefferson Airplane, the Mamas and the Papas, the Grateful Dead, Elvis Presley, Frank Sinatra, the Monkees, Liverpool Five, Love, The Seeds, The Collectors, Crosby Stills Nash and Young, the Jackson 5, Leo Kottke, Seals and Crofts, the Blackbyrds, Sweetwater, and George Strait.