On 11 November 1922, at St Brigid's Church, Marrickville, he married Esmey Mary Kelly and they were to have three sons and two daughters. When their first son was born on 12 February 1924, they named him "Thomas James" after Cahill's younger brother. In 1922, after taking part in an unsuccessful attempt to unseat the Engineers Union's NSW governing body through a federal council, Cahill among several others was sued by the union for "illegally holding office within the union". Although the suit was later dismissed by the Chief Judge in Equity, Philip Street, Cahill was banned from holding office in the union by the AEU until 1925.