Thatcher and his twin sister, Carol Thatcher, were born six weeks prematurely by caesarean section on 15 August 1953, the same year that their mother qualified as a barrister. Their early years were spent in Chelsea, London. Their mother was narrowly defeated in her bid to become the Conservative Party candidate in the Orpington by-election, 1955. She was first elected to Parliament at the 1959 general election. The children, aged six at the time, featured in her first television interview. His sister observed: "All my childhood memories of my mother were just someone who was superwoman before the phrase had been invented. She was always flat out, she never relaxed, household chores were done at breakneck speed in order to get back to the parliamentary correspondence or get on with making up a speech."
Mark was sent to board at Belmont, Mill Hill preparatory school at the age of eight and then to Harrow School, which he left in 1971 having passed three O-level exams. He went on to study accountancy but failed his accountancy exams with Touche Ross on three occasions.
The children were sixteen years old when their mother entered the Cabinet for the first time as Secretary of State for Education and 25 when she became Prime Minister at the 1979 general election. In January 1982, Thatcher caused international embarrassment to his mother. He became lost for six days in the Sahara Desert while competing in the Dakar Rally (see below). His father flew to Dakar, and at the height of the search there were nine aircraft from Algeria, France and the UK involved, in addition to the Algerian land forces. The Prime Minister insisted on paying £2,000 personally towards the cost of the search, and an unpaid 11,500 dinar hotel bill, one third of which was for drinks, caused diplomatic embarrassment. After this incident, there were headlines about possible conflicts of interest between his business interests and his mother's political visits; including the suggestion that he had received millions of pounds in commission in relation to the Al-Yamamah arms deal (see Career section below). Sir Bernard Ingham, the Prime Minister's press secretary, suggested that he could best help the government win the 1987 general election by leaving the country. Margaret Thatcher's biographer, David Cannadine, stated that Mark Thatcher "traded shamelessly on his mother's name" and that he "continued to attract controversy and investigation from the tax authorities", much to his mother's embarrassment.
Thatcher moved to Dallas, Texas, United States in the mid-1980s, where he met his first wife, Diane Burgdorf (now wife of James Beckett), in 1987. Their first child, Michael Thatcher, was born in 1989, and their second child, Amanda Margaret Thatcher, was born in 1993. In 1992, he became The Honourable Mark Thatcher when his mother was made a life peer. In 1996, he moved to South Africa following financial scandals in the United States.
In 2003, he became the Honourable Sir Mark Thatcher when he succeeded to the hereditary Thatcher Baronetcy awarded to his father in 1990. He and his wife announced their intention to divorce in September 2005 after eighteen years of marriage. His wife moved back to the United States with their children, the same year that he pleaded guilty in relation to an attempted coup in Equatorial Guinea (see below) and questions were being asked in the government about whether he should be stripped of his title.
Following his guilty plea and his divorce, he left South Africa in 2005 for Monaco on a one-year temporary residency permit, while his wife and children returned to the United States. Thatcher was unable to get a US visa due to his South African conviction and remains barred from entering the United States.
His Monaco residency was not renewed as he was said to be on a list of "undesirables" who would not be allowed further residency and he was required to leave by mid-2006. He was refused residency in Switzerland and settled in Gibraltar, where he married his second wife, Sarah Jane Russell, in March 2008. Russell is the daughter of Terry Clemence, a property developer and sister to the Viscountess Rothermere. She was formerly married to Lord Francis Russell, the younger son of John Russell, 13th Duke of Bedford.
He was in Barbados when he received news of his mother's death. He returned to the UK to act as chief mourner at his mother's funeral, which took place at St Paul's Cathedral, London on 17 April 2013.
In April 2016, Thatcher was named in the Panama Papers scandal.