"The Diary" was written by Neil Sedaka and Howard Greenfield. It was Sedaka's first hit, and one of the first successes for the brand new record label Aldon Music.

Rich Podolsky's book Don Kirshner: The Man with the Golden Ear tells the fascinating story behind this hit. Earlier, when Sedaka and Greenfield were working with Connie Francis on her hit "Stupid Cupid," Connie became distracted and began scribbling in her diary. Howard Greenfield began teasing her about it, wanting to sneak a peek, but Connie refused. Greenfield went home from the sessions with the idea for the next song in his head.

Greenfield and Sedaka originally agreed to write this song for the 1950s doo-wop group Little Anthony and the Imperials. Don Kirshner and Al Nevins of Aldon, flush from their success with "Stupid Cupid," referred them to their peer George Goldner, producer for End Records. Goldner had the Imperials record it, but when the demo came back, Sedaka was crestfallen because it wasn't done the way he'd have liked. Goldner and Sedaka decided that Sedaka should take the song back to Aldon and record it himself.

Nevins, who had formerly led his own band The Three Suns, had connections from that experience with RCA Records, so he took Sedaka over there and convinced them to let him do a session. Sedaka shook hands with Steve Sholes, an executive who was fresh off signing Elvis for RCA Victor.

The first recording for "The Diary" sounded "too polished," according to Nevins, so he rescheduled Sedaka to record again the next day to get the sound he liked.

Neil Sedaka got to debut the song himself on the TV show American Bandstand. His success led the 19-year-old Sedaka to become the proud owner of a Chevy Impala convertible, in which he would zip down the highway and, as recalled in Podolsky's book, occasionally hear his own song come on the radio.