He worked as a music journalist in the 1960s under the name Johnny Dean, and was the founder and editor of Beat Instrumental, The Beatles Book, and Record Collector magazines.
In the early 1960s he became the advertising manager of Pop Weekly, a magazine published by Robert Stigwood, through which he first came into contact with Brian Epstein.
[1] As the Beatles' popularity in the UK grew, O'Mahony approached Epstein with the suggestion that he produce a monthly magazine devoted to the band.
As Johnny Dean, O'Mahony continued to edit and write for the magazine with the full support of Epstein and the band, and also launched the parallel Monkees Monthly in early 1967.
[1] He also continued to write, and as John Dean wrote the songs and script for the 1999 musical film Julie and the Cadillacs.