Thurso is notable for having served in the House of Lords both before and after a period in the House of Commons. He first joined Parliament in the Lords as a hereditary peer between 1995 and 1999. Most hereditary peers were removed from Parliament following the House of Lords Act 1999. Subsequently, he was elected Member of Parliament (MP) for Caithness, Sutherland and Easter Ross from the 2001 election until he was defeated in the 2015 election by SNP candidate Paul Monaghan. He was the fifth generation of the Sinclair family to represent the Caithness area in the House of Commons. In 2016, he returned to the House of Lords after winning a by-election to fill a vacancy among the remaining Liberal Democrat hereditary peers.
John Sinclair was educated in Thurso and at Eton College. Thurso joined the Savoy Group as a management trainee in 1972 and following this worked for many years in the hospitality industry. He was a manager at the Lancaster Hotel in Paris (1981–85) and founding the hotel at Cliveden (1985–92) before becoming CEO of Granfel Holdings, owners of East Sussex National Golf Course from 1992 to 1995. Finally from 1995 until his election to parliament in 2001 he was CEO of the Champneys Group. During his time in this job he featured in the TV documentary Trouble at the Top - Shape up with Lord Thurso.
Lord Thurso comes from a family of Liberal parliamentarians. The former constituency of Caithness and Sutherland had been held by his grandfather, Archibald Sinclair from 1922 until 1945. Archibald Sinclair was the 1st Viscount Thurso and a Liberal Party leader. Thurso has been married to Marion for 26 years and they have a daughter and two sons. The family live at Thurso, Caithness.
Thurso holds the Presidency of The Tourism Society and the Academy of Food and Wine Service. He is a Fellow of Confederation of Tourism and Hospitality (HCIMA) (FIH) and served as its Patron for six years, until June 2003. He was President of the British International Spa Association, a Trustee of the Clan Sinclair Trust, and Patron of the Bluebell Railway 50th Anniversary Appeal. In that capacity, on 24 April 2009, at the railway's Horsted Keynes station he carried out the ceremonial renaming of the Battle of Britain class locomotive named after his grandfather, Sir Archibald Sinclair, Secretary of State for Air during that battle. On 7 March 2016, it was announced that Lord Thurso would become the chair of VisitScotland. In August 2017, he was appointed as Lord Lieutenant of Caithness.
Following his father’s death in 1995 he took his seat in the House of Lords as the 3rd Viscount Thurso where he became spokesman on Tourism and later Food Matters. Thurso spoke many times in the House of Lords in favour of Lords reform. His automatic right as a hereditary peer to sit in the House of Lords was abolished in 1999, and he did not attempt to remain in that capacity. At the 2001 general election he was elected to the House of Commons to represent Caithness, Sutherland and Easter Ross.
He served as Liberal Democrat Shadow Scotland Secretary under Charles Kennedy, but was sacked by Sir Menzies Campbell. He has publicly gone against party policy by declaring his support for nuclear power, and his criticism of 24-hour drinking and wind power.