In 1948 he joined The Bulletin to write the "Personal Items" page. He apparently contributed articles about wartime issues to The Bulletin and other articles of a similar type certainly appeared in The Australasian Post, The Australian Journal, The Sydney Morning Herald, Woman, The Australasian, The Adelaide Advertiser, The Western Mail and Pocket Book Weekly. Stories by MacDonnell also appeared in the annual Australian War Memorial books that were issued during and after World War 2.

His first book, Fleet Destroyer - a collection of stories about life on the small ships - was published by The Book Depot, Melbourne, in 1945 while MacDonnell was on active service as a Petty Officer.

Constable London published Macdonnell's first novel, Gimme the Boats, in 1953, followed in the same year by Wings off the Sea and subsequently by Jim Brady, Leading Seaman (1954), Commander Brady (1956) and Subsmash (1960). Macdonnell began writing full-time for Horwitz in 1956, writing an average of a dozen books a year.

In 1988, he retired to Buderim on the Sunshine Coast in Queensland his wife Valerie in 1988. He died peacefully in his sleep at a Buderim hospital in 2002. He is survived by his children Beth, Jane and Peter.

Macdonnell's naval stories feature several recurring characters: Captain "Dutchy" Holland, D.S.O., Captain Peter Bentley, V.C., Captain Bruce Sainsbury, V.C., Jim Brady, and Lieutenant Commander Robert Randall.