Sidey was born on 27 May 1863, to John and Johan Murray Sidey, in the Dunedin suburb of Corstorphine. His father had come to wealth during the Otago Gold Rush as a storekeeper. Tom Sidey attended Otago Boys' High School and graduated from the University of Otago with a law degree (LLB) in 1889. In the following decade, he worked as a solicitor.
He married Helena (née Baxter) on 17 June 1903. They had one son.
|Parliament of New Zealand|
Sidey was a member of the Caversham Borough Council. He was elected Mayor of Caversham on three occasions: in 1894, 1899 and 1901.
He was elected to the House of Representatives in the Caversham by-election as an independent liberal in 1901. The by-election was caused by the death of Arthur Morrison. Sidey joined the Liberal Party as part of its left (radical) wing, and stayed with the party until the end.
He represented the Caversham electorate from 1901 to 1908, and then the Dunedin South electorate from 1908 to 1928, when he retired. He was then appointed to the Legislative Council from 1928 until 1933.
He put forward a private member's bill for putting clocks forward an hour in summer every year from 1909. It was nearly passed in 1915. It was passed in the House of Representatives but rejected by the Legislative Council in 1926. It was finally approved in 1927.
Sidey died at home on 20 May 1933. He was survived by his wife and son, Thomas Kay Stuart Sidey. His son became Mayor of Dunedin from 1959–65. His widow, Helen, Lady Sidey, was appointed an Officer of the Order of the British Empire for social welfare services, especially in connection with women's organisations, in the 1953 Coronation Honours.
- Olssen, Erik (updated 22 June 2007). "Sidey, Thomas Kay 1863 - 1933". Dictionary of New Zealand Biography. Retrieved 21 July 2010.
- Wilson, James Oakley (1985) [First ed. published 1913]. New Zealand Parliamentary Record, 1840–1984 (4th ed.). Wellington: V.R. Ward, Govt. Printer. OCLC 154283103.
- "Deaths". Otago Witness. Issue 2489, 27 November 1901. p. 43. Retrieved 21 July 2010.
- The London Gazette: . 26 May 1953. Retrieved 14 June 2013.