Thatcher was born in Lewisham, London, as the first-born child of a New Zealand-born British businessman, Thomas Herbert "Jack" Thatcher. At age eight he entered a preparatory school as a boarder in Bognor Regis, following which he attended the nonconformist public school, Mill Hill School. At school he excelled at cricket, being a left-handed batsman. Thatcher left Mill Hill at age 18 to join the family paint and preservatives business, Atlas Preservatives. He also studied accountancy to improve his grasp of business, and in 1935 was appointed works manager. He joined the Territorial Army shortly after the Munich crisis, as he was convinced war was imminent: a view reinforced by a visit he made to Germany with his father's business in 1938.
During the Second World War, Thatcher was commissioned as a second lieutenant into the 34th Searchlight (Queen's Own Royal West Kent Regiment) of the Royal Engineers. He transferred to the Royal Artillery on 1 August 1940. During the war he was promoted to war substantive captain and temporary major. Although he saw no real fighting – despite serving through the Invasion of Sicily and the Italian Campaign – he was twice mentioned in dispatches, and in 1945 was appointed a Member of the Order of the British Empire (MBE). The first mention in dispatches came on 11 January 1945, for service in Italy, and the second on 29 November 1945, again for Italian service. His MBE was gazetted on 20 September 1945, and was awarded for his efforts in initiating and supporting Operation Goldflake, the transfer of I Canadian Corps from Italy to the north-west European theatre of operations. By this time Thatcher was based in Marseille, attached to HQ 203 sub-area. In the recommendation for the MBE (dated 28 March 1945), his commanding officer wrote: "Maj. Thatcher set an outstanding example of energy, initiative and drive. He deserves most of the credit for [...] the excellence of the work done." He also received the French approximate equivalent of a mention when he was cited in orders at Corps d'Armée level for his efforts in promoting smooth relations between the Commonwealth military forces and the French civil and military authorities. He was promoted to substantive lieutenant on 11 April 1945. Demobilised in 1946, he returned to run the family business, his father having died, aged 57, on 24 June 1943, when Thatcher was in Sicily. Because of army commitments, Thatcher was unable to attend the funeral.
He remained in the Territorial Army reserve of officers until reaching the age limit for service on 10 May 1965, when he retired, retaining the honorary rank of major.
On 21 September 1982 he was awarded the Territorial Decoration (TD) for his service.
Denis Thatcher married twice, during the Second World War to Margaret Doris Kempson in 1942 (divorced 1948), and in 1951 to Margaret Thatcher (née Roberts).
On 28 March 1942, Thatcher married Margaret Doris Kempson (23 January 1918 – 8 June 1996), the daughter of Leonard Kempson, a businessman, at St. Mary's Church, Monken Hadley. They had met at an officers' dance at Grosvenor House the year before.
Although initially very happy, Thatcher and his first wife never lived together. Their married life became confined to snatched weekends and irregular leaves as Thatcher was often abroad during the war. When Thatcher returned to England after being demobilised in 1946, his wife told him she had met someone else and wanted a divorce. Their childless marriage ended in the first weeks of 1948. Kempson married Sir (Alfred) Howard Whitby Hickman, 3rd Baronet (1920–1979) on 24 January the same year. Thatcher was so traumatised by the event that he completely refused to talk about his first marriage or the separation, even to his daughter, as she states in her 1995 biography of him. Thatcher's two children found out about his first marriage only in February 1976, by which time their mother was Leader of the Conservative Party, and only when the media revealed it.
In February 1949, at a Paint Trades Federation function in Dartford, he met Margaret Hilda Roberts, a chemist and newly selected parliamentary candidate. When she met Denis for the first time she described him as "not a very attractive creature – very reserved but quite nice". They married on 13 December 1951, at Wesley's Chapel in City Road, London: the Robertses were Methodists. Margaret Thatcher was elected Leader of the Conservative Party in 1975 and won the 1979 general election to become the first female Prime Minister in British history. Denis Thatcher was the first husband of a British prime minister.
In 1953, they had twin children (Carol and Mark), who were born on 15 August, seven weeks premature. Denis was watching the 1953 Ashes series at the time.
Not long after the 1964 general election, Denis Thatcher suffered a nervous breakdown which put a severe strain on their marriage. The breakdown was likely caused by the increasing pressure of running the family business, caring for his relatives, and his wife's preoccupation with her political career, which left him lonely and exhausted. Thatcher sailed to South Africa and stayed there for two months in order to recuperate. His wife's biographer David Cannadine described it as "the greatest crisis of their marriage", but he eventually did return home and sold the family company on very good terms, which relieved him considerably.
This second marriage for Thatcher led to the future prime minister being sometimes referred to as "Mrs Denis Thatcher" in such sources as selection minutes, travel itineraries, and society publications such as Queen, even after her election as a parliamentarian. As Margaret's political career progressed, she preferred to be known only as "Mrs Thatcher".
Thatcher was already a millionaire when he met Margaret and financed her training as a barrister, and a home in Chelsea; he also bought a large house in Lamberhurst, Kent, in 1965. His firm employed 200 people by 1957.
Thatcher became managing director of Atlas in 1947 and chairman in 1951, and led its overseas expansion. By the early 1960s he found being in sole control of the family company difficult; this, his wife's political career, and their desire for financial security caused Thatcher to sell Atlas to Castrol in 1965 for £530,000 (£9,255,000 today). He continued to run Atlas and received a seat on Castrol's board; after Burmah Oil took over Castrol in 1966 Thatcher became a senior divisional director, managing the planning and control department. He retired from Burmah in June 1975, four months after his wife won the Conservative Party leadership election.
In addition to being a director of Burmah Oil Thatcher was vice-chairman of Attwoods plc from 1983 to January 1994, a director of Quinton Hazell from 1968 to 1998, and a consultant to AMEC plc and CSX Corp. He was also a non-executive director of retail giant Halfords during the 1980s.
Thatcher refused press interviews and made only brief speeches. When he did speak to the press, he called his wife "The Boss". She often acknowledged her husband's support. In her autobiography, Margaret wrote: "I could never have been Prime Minister for more than 11 years without Denis by my side." Thatcher saw his role as helping her survive the stress of the job, which he urged her to resign on the 10th anniversary of her becoming Prime Minister, in 1989, sensing that otherwise she would be forced out (as happened a year later).
In an interview with The Times in October 1970, Thatcher said: "I don't pretend that I'm anything but an honest-to-God right-winger—those are my views and I don't care who knows 'em." His public image was shaped by the satirical "Dear Bill" columns appearing since 1979 in Private Eye, which portrayed him as a "juniper-sozzled, rightwing, golf-obsessed halfwit", and Thatcher found it useful to play along with this image to avoid allegations of unduly influencing his wife in political matters.
Given his professional background Thatcher served as an advisor on financial matters, warning Margaret about the poor condition of British Leyland after reviewing its books. He often insisted that she avoid overwork, to little avail, sometimes pleading "Bed, woman!" They otherwise usually kept their careers separate; an exception was when Thatcher accompanied his wife on a 1967 visit to the United States sponsored by the International Visitor Leadership Program.
Thatcher was consistently strongly against the death penalty, calling it "absolutely awful" and "barbaric". Not unlike his wife, Thatcher was consistently anti-socialist. He told his daughter in 1995 that he would have banned trade unions altogether in Britain. He had a low regard for the BBC, thinking it was biased against his wife and her government, as well as unpatriotic. In his most famous outburst about the corporation, he claimed his wife had been "stitched up by bloody BBC poofs and Trots" when she was questioned by a member of the public about the sinking of the ARA General Belgrano on Nationwide in 1983.
Thatcher was reported by New Zealand broadcaster and former diplomat Chris Laidlaw – at the time NZ High Commissioner to Zimbabwe – as leaning towards him during a Commonwealth Heads of Government Meeting, asking "So, what do you think the fuzzy wuzzies are up to?"
In December 1990, less than a month after his wife resigned her office after over a decade, it was announced that Denis Thatcher would be created a baronet (the first such creation since 1964). The award was gazetted in February 1991, giving his title as Sir Denis Thatcher, 1st Baronet, of Scotney in the County of Kent. Thus, his wife, the former prime minister, was entitled to be known as Lady Thatcher (while retaining her seat in the House of Commons); however she made it known that she preferred "Mrs Thatcher", and did not use the title. She was created a life peeress as Baroness Thatcher (thus Lady Thatcher, in her own right) upon her retirement from the Commons after the 1992 general election.
Thatcher's baronetcy was a hereditary title that was to be inherited by his son after his death. His was the first British baronetage to be granted since 1964, and no baronetages have been created thereafter.
On 17 January 2003, Denis Thatcher underwent a six-hour heart-bypass operation. He had complained of breathlessness in the weeks before Christmas 2002 and the problem was diagnosed in early January. He left hospital on 28 January 2003, and appeared to have made a full recovery. He visited his son Mark in South Africa in April, but by the middle of June, by which time he had turned 88, he again complained of breathlessness and was taken to hospital. There pancreatic cancer was diagnosed, along with fluid in his lungs. He died on 26 June at Westminster's Lister Hospital in London. Denis and Margaret Thatcher had been married for almost 52 years.
His funeral service took place on 3 July 2003, at the chapel of the Chelsea Royal Hospital in London, followed by a cremation at Mortlake Crematorium in Richmond, London. On 30 October a memorial service was held at Westminster Abbey. His ashes were buried under a white marble marker just outside the Royal Hospital in Chelsea. His wife's ashes were buried with his following her death in 2013.Thatcher's single public interview, which took place in October 2002, was released as a DVD, Married to Maggie, after his death. In it he called his wife's successor, John Major, a "ghastly" prime minister and said it would have been good had he lost the 1992 general election. He added that he thought his wife was the "best prime minister" since Winston Churchill.
Below the Parapet – The Biography of Denis Thatcher by Carol Thatcher (his daughter). Published by Harper Collins in 1996. In it, Thatcher said that politics as a profession or way of life did not appeal to him. World leaders he personally got on with were George H. W. Bush, F. W. de Klerk, King Hussein of Jordan and Mikhail Gorbachev, whilst he disliked Indira Gandhi and Sonny Ramphal. He revealed that spouses he personally liked were Raisa Gorbacheva, Nancy Reagan and Barbara Bush. Thatcher said that he was not sure where the Falkland Islands were until the invasion occurred in 1982.