Sir Mark Thatcher, 2nd Baronet (born 15 August 1953) is the only son of Sir Denis Thatcher and Margaret Thatcher, the former British Prime Minister, and twin brother of Carol Thatcher. In addition to his prominence as the only son of one of the world's best known politicians, Thatcher has attracted headlines for his early youthful playboy lifestyle, involvement in motorsports, business associations, and for the role he played in an attempted coup in Equatorial Guinea, for which role he was fined three million rand (approximately $500,000) and received a four-year suspended jail sentence.

Thatcher was educated at Harrow School from January 1967 to 1971, where his nickname was "Thickie Mork". He became an articled clerk at Touche Ross, a City of London firm of Chartered Accountants, but did not succeed in becoming an accountant.

Thatcher married Diane Burgdorf, the conservative Lutheran daughter of the millionaire Texas car dealer Theodore C. Burgdorf, on 14 February 1987 in Queen's Chapel of the Savoy, London, England. They reportedly met at a party for D Magazine, a Dallas lifestyle publication, while Thatcher was living in Texas as a representative of the luxury automotive company Lotus Cars. The family moved to South Africa possibly to avoid bad publicity because of allegations against Mark Thatcher of racketeering that resulted in a £4 million civil action in 1994. They have a son and a daughter: