That same year she married the budding Australian author D'Arcy Niland (1917–1967), with whom she had been corresponding as pen pals for some years, and whom she had finally met on a previous visit to Sydney. There she embarked on a career as a freelance writer. Park and Niland had five children, of whom the youngest, twin daughters Kilmeny and Deborah, went on to become book illustrators. (Park was devastated when Niland died in Sydney at the age of 49 from a heart ailment; Kilmeny also predeceased her — see the "Herald" obituary.) Park had eleven grandchildren and five great-grandchildren. The writer Rafe Champion is her son-in-law. In addition, Darcy Niland's brother Beresford married Ruth Park's sister Jocelyn.