He was born in
Alloa,
Clackmannanshire, Scotland, the youngest son of Andrew Thomson, a timber merchant, and his wife, Janet Mitchell. He was a maternal grandson of
William Mitchell (1781–1854). He was educated at the
Edinburgh Institution. He followed in his father's trade as a timber merchant and monies (mainly from his mother's side) allowed him to purchase major tracts of land in
Kincardineshire and
Peeblesshire. His firm, Mitchell-Thomson & Co, operated from
Granton Harbour north of
Edinburgh. ) He entered local politics in 1882 standing unsuccessfully for a council seat in
Edinburgh. He finally was elected as a councillor in 1890. He served on the city’s Gas, Education and Water Commissions. He was the chairman on the Northhill Soup Kitchen committee in Edinburgh. He was a trustee and chairman for
George Heriot’s School in Edinburgh. He was Provost of Edinburgh (1897–1900) and a
JP for Peebleshire. He was also a representative for Edinburgh to the General Council of the
Church of Scotland. He served on the committee of the Edinburgh branch of the Navy league in the 1900s. He was not a free trader in that he was chairman of the Scottish Trade Protection Society (1890s) and later the
Tariff Reform League (1900s). He served as Lord Provost of Edinburgh from 1897 to 1900, succeeding
Sir Andrew McDonald. As was customary for retiring Lord Provost's, he was created a
baronet by
Queen Victoria in 1900. During this period (1899) he was also elected a
Fellow of the Royal Society of Edinburgh. His proposers were
Sir John Murray,
Alexander Crum Brown,
Robert Flint and
Alexander Buchan. During this period he also served as Honorary President of George Heriots Former Pupils Golf Club. He started out a partner in the family timber business and later held directorships at various times in a range of Scottish-based companies (the
Bank of Scotland, the Scottish Widow’s Fund Life Assurance Society, the British Investment Trust Company Arizona Trusts and Mortgage Company, the Scottish Reversionary Company Ltd.; the
Caledonian Railway and the London Advisory Committee of the Canada Steamship Line Limited (1917). By 1916 he had acquired over of land including an estate in
Peeblesshire called
Polmood which he sold in 1917. He also owned land in
Kincardineshire. In later life he was living at 6
Charlotte Square, a huge and prestigious townhouse in the centre of Edinburgh designed by
Robert Adam. His "country estate" was
Polmood House in
Tweedsmuir. He died at home in Edinburgh on 15 November 1918. He is buried facing the north path of
Dean Cemetery in Edinburgh, at the west end of the section closest to the main entrance. ==Family==