He was the director of the Burmah Oil Trading Co., chairman of the Atlas Preservative Co., vice-chairman of Attwoods plc, director of Quinton Hazell plc, and consultant to Amec plc and CSX Corp.
Educated at Mill Hill Boarding School, he left at the age of 18 to join the family business, Atlas Preservatives. During the war he served in the 34th Searchlight (Queen's Own Royal West Kent) regiment of the Royal Engineers before being promoted to the rank of Major. He was twice mentioned in dispatches and in 1945 was awarded an MBE.
In 1942 he married Margaret Kempson. They were divorced in 1948.
In 1950 while attending a Conservative Party function he met Margaret Hilda Roberts, a lawyer and British politician who later became the first female Prime Minister. They were married the next year. In rare communications with the press, he referred to her as "The Boss".
The public perception of his character was formed chiefly from a series of spoof letters published in the satirical magazine Private Eye in the 1980s. The "Dear Bill" column written by Richard Ingrams and John Wells took the form of a letter purported to be from Denis to his real life friend and golfing partner Bill Deedes (former editor of The Daily Telegraph), detailing life at Number 10. The letters portrayed Denis Thatcher as a reactionary interested only in golf and gin. John Wells used the character portrayed in the letters, and created the stage play Anyone For Denis (also shown on television).
Margaret Thatcher however, often acknowledged her husband's support. In her autobiography she wrote "I could never have been prime minister for more than 11 years without Denis by my side".
Denis Thatcher was created a Baronet in 1991, and his wife was created Baroness Thatcher of Kesteven in 1992.
Their children are Sir Mark Thatcher and Carol Thatcher.
He died at the age of 88 at London's Lister Hospital, five months after undergoing heart bypass surgery, with his funeral on July 3, 2003 at the chapel of the Royal Hospital in Chelsea.
Prime Minister Tony Blair called him a "kind and generous-hearted man, a real gentleman who had many friends here and abroad."
Ulster Unionist David Burnside recalled a reception in Blackpool "to which Sir Denis came along with his minder and declared: "I don't know what reception I'm at but for God's sake give me a gin and tonic."