Thomas Kay Sidey was born at the home of his parents, John Sidey and his wife, Johan Murray, at Corstorphine, Dunedin, New Zealand, on 27 May 1863. John Sidey had made a substantial sum as a storekeeper during the goldrushes, and had then greatly enlarged his Caversham farm and invested shrewdly in property. Tom received his early education from a tutor, but later attended two private academies and then Otago Boys' High School. In the early 1880s he studied at the University of Otago for his BA and then took law, graduating LLB in 1889. In the 1890s he established himself as a solicitor.
Sidey soon began a life of public service in Dunedin and Caversham. He had begun teaching sabbath school during the 1880s and was active in the Sunday school movement. He joined an Oddfellows lodge and the Dunedin Horticultural Society, and served on the Caversham School Committee (1890–1901). He also played an active part in the Caversham Harrier Club and a literary and debating society. Sidey was elected to the Caversham Borough Council in 1890 and became mayor of Caversham in 1894. He was a member of the Council of the University of Otago and the Otago Boys' and Girls' High Schools Board. In 1897–98 he took a trip to Great Britain armed with letters of introduction and a camera. On his return he gave countless talks accompanied by lantern slides. He was again elected mayor in 1899 and 1901. He married Helena Baxter at Caversham on 17 June 1903; they were to have one son.
In 1901 Sidey, standing as an independent Liberal, narrowly won the Caversham parliamentary seat in a by-election; two candidates, including William Earnshaw, MHR from 1890 to 1896, split the crucial labour vote. Despite his reputation for indecisiveness, Sidey quickly obtained positions on the education, railways and labour committees and identified publicly (if cannily) with the left wing of the Liberal caucus on industrial issues and the increasingly contentious matter of the cost of living. In 1905 he delighted radicals by seconding a motion to nationalise the food supply and the clothing industry.
Sidey worked assiduously to help his constituents. Men from the Hillside railway workshops, the largest employer in his electorate, wrote frequently with their grievances. He dealt with all such cases promptly, courteously and effectively. Whenever the minister of railways proved insufficiently responsive, Sidey joined the other Liberal members from similar constituencies to embarrass his government in public.
In Parliament Sidey fought for the New Zealand Local Time Bill, a proposal to turn the clocks forward by an hour and increase the time available for sport and gardening in the evening. It enjoyed great support in Caversham and on The Flat in South Dunedin. He also worked hard to promote the professionalisation of music teachers and dentists. This latter crusade became his major concern during his first two terms and he eventually secured a new system of professional accreditation and a dental school for the university. He then set about instituting a similar system of professional self-government for lawyers.
Sidey's attention to his electorate led to his easily defeating Earnshaw in the election of 1905. He was not especially prominent in Parliament, although he pursued his strong interest in education by introducing a bill to authorise a plebiscite on the issue of Bible reading in schools. As he strongly favoured a secular education system this was doubtless done to placate Presbyterian constituents, but new boundaries for his seat, incorporating South Dunedin, soon made him wary of antagonising his new Catholic electors.
In 1908 Sidey's most serious challenge for the new Dunedin South seat was from the Political Labour League of New Zealand; he won easily. Three years later, however, he faced a stronger challenge from the organising secretary for the Dunedin and Suburban General Labourers' Union, 'Big Jack' MacManus, who had helped form the Dunedin branch of the New Zealand Socialist Party. The two men nicely symbolised the issues of class. Sidey lived in the family's mansion in Corstorphine, overlooking Caversham; MacManus, in one of the rougher areas of South Dunedin. MacManus was Irish and Catholic, a pugilist who weighed in at 18 stone; Sidey, a neatly dressed and self-effacing Presbyterian Sunday school superintendent. The Otago Daily Times damned both candidates as socialists but Sidey won narrowly.
Sidey had won a reputation as a good committee man and a good constituency man, and his kindliness and courtesy won the respect and affection of colleagues. These virtues, however, failed to win him a place in cabinet. Perhaps in frustration he vigorously but unsuccessfully sought the position of vice chancellor of the University of Otago. Nor did he win selection to the wartime National government in 1915. In 1919 he wanted to resign because his health had deteriorated, but he stood for re-election to meet the challenge of the New Zealand Labour Party. He defeated J. T. Paul by 84 votes and in the 1920s became a senior member in the dying Liberal party. In 1921 Sidey eventually became vice chancellor, and battled to persuade the university to award a degree in theology; he became chancellor in 1925. He also sat on the Council of Knox College, and in 1927 finally secured enactment of his daylight saving measure, as the Summer Time Act.
In 1928 Sidey decided not to seek re-election to Parliament. When Sir Joseph Ward led the United Party to an unexpected victory, however, Sidey agreed to lead the government in the Legislative Council and finally entered cabinet as attorney general; a year later he was also minister of justice. In 1930 he represented New Zealand at the Imperial Conference and obtained a clause in the Statute of Westminster which exempted New Zealand from its operation until the New Zealand Parliament enacted the measure. When Ward's health deteriorated some rumours identified Sidey as a successor. It is not clear whether he wanted the job, but his gracious acceptance of George Forbes may have produced his knighthood in 1930. Loyal to the end, in 1931 he resigned from cabinet to allow Forbes to form a coalition government. He died at his home on 20 May 1933, survived by his wife and son. Sidey was remembered for his loyalty and service to the many causes he espoused, but had, perhaps, lacked the driving ambition necessary to a more substantial political career.