John Tiffin Stewart was born at Rothesay, Bute, Scotland, on 18 November 1827. He was the son of Agnes Oliphant and her husband, John Stewart, commander of the fishery cutter Swift. Stewart was educated locally at Rothesay. He was then apprenticed to Gordon and Hill, then Bell and Miller, civil engineers of Glasgow, before attending the University of Glasgow and graduating as a civil engineer.
In 1852 Stewart left Scotland for Melbourne, Australia, and worked for three years on the Ovens goldfields in Victoria. Coming to New Zealand in 1855 he was immediately appointed to the government engineering staff. In 1857 he mapped the Manawatu River and its tributaries from the sea to the gorge. In October 1858 he was appointed assistant surveyor in the Land Purchase Department of the Government, his work being to define native land boundaries. From 1861 until 1863, as provincial engineer for Wellington, he surveyed roads in Wairarapa and around Castlepoint.
In January 1864 Stewart took charge of the Wellington and Manawatu districts with headquarters at Foxton. During the next four years he surveyed the Maori land purchased by the government in the Waitotara and Manawatu areas. He supervised the subdivision of Palmerston North, Feilding, Halcombe and Rongotea, giving all of them large town-squares. He also planned and saw the completion of the Manawatu Gorge road. On 22 November 1865 he married Frances Ann Carkeek at Wellington; they were to have five daughters and five sons.
Stewart worked on the Wanganui town bridge between 1868 and 1871, adapting George Stephenson's plan, sent out from England. As the design for the bridge was too short, he added a wooden extension at one end. He was known to roll up his plans in an oil sheet and walk down the coast to Wellington to confer with his superiors, and walk back again.
In November 1870 Stewart was appointed district engineer, Public Works Department, at Foxton, in charge of the Taranaki, Wanganui and Hawke's Bay districts. In 1885 he transferred to Wanganui. He was immediately elected to the borough council, serving a two-year term. In December 1885 he prepared a comprehensive report on snagging and clearing the Wanganui River for navigation. He retired as district engineer in 1889, and in 1892 became the government appointee to the newly created Wanganui River Trust. He served as chairman from 1892 to 1898; as secretary, with the additional responsibility of being in charge of works, from 1898 to 1900; and as honorary engineer from 1900 to 1909. During this period the river was made navigable as far as Taumarunui and became part of a scenic route which took tourists to the thermal areas of the central North Island.
Stewart played an active role in the Wanganui community. He was vice president of the Wanganui Public Museum board of trustees for 20 years and president for one year. He served as president of the Wanganui Bowling Club. He and Frances Stewart were among the founders of the Wanganui Orphanage in 1889, and, impressed by the work of Truby King, they bequeathed their house for use as a Karitane home for sick infants. When the Wanganui Borough Council promoted a competition for the development of Lake Virginia in 1904 Stewart, in co-operation with his daughter and son-in-law, submitted the winning plan. He also found time to write monthly astronomical and rainfall notes for publication in the Wanganui Chronicle, and was a skilled watercolour artist.
John Tiffin Stewart died at Wanganui on 19 April 1913. He was survived by his wife, who died on 12 November 1916, and nine of their children.