It is considered by some to be the most important and pivotal of his early compositions because his performance of the song at an April 15, 1972, live radio concert at Sigma Studios on WMMR in Philadelphia, and the subsequent airplay this live version received on the station, brought him to the attention of major record labels, including Columbia, with whom he would sign a recording contract in 1973.

Joel wrote "Captain Jack" in late 1971, while sitting in his apartment in Oyster Bay, Long Island, looking out the window, trying to find inspiration for a song. Across the street was a housing project, and he observed suburban teenagers going into the project and obtaining heroin from a dealer known as "Captain Jack". "It's about coming out of the New York suburbs," Joel told John Kalodner in 1974. "But in my travels I have seen a lot of the same suburb all over the country. The song is sort of brutal, but sometimes it is good to be brutal and offend people—it keeps them on their toes."

The song, according to Joel, is an anti-drug song. He says, "What's so horrible about an affluent young white teenager's life that he's got to shoot heroin? It's really a song about what I consider to be a pathetic loser kind of lifestyle. I've been accused of, 'Oh, this song promotes drug use and masturbation.' No, no, no. Listen to the song. This guy is a loser." In writing about the song in the liner notes of his Songs in the Attic album, Joel once again emphasized the point: "...so many friends shoveled under the Long Island dirt. The miracle of modern chemistry killed them if Vietnam didn't."

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Wikipedia