The Tory grandee masterminded the general election campaign that delivered her thumping majority in 1983, and held a series of senior posts in her governments.
But the revelation that he had a love child with a former secretary curtailed his career, ensuring that he never secured one of the great offices of state – despite Thatcher’s apparent wish that he should be her successor.
Prime Minister David Cameron led tributes to the former Cabinet minister, saying he had been “part of an extraordinary political generation”.
As a grammar school pupil who went to Cambridge, becoming a chartered accountant and enjoying a successful business career before entering parliament, Cecil Parkinson had much in common with Baroness Thatcher.
According to Charles Moore’s recent authorised biography of the former prime minister, Parkinson was her “favourite” in the cabinet and she wanted to make him foreign secretary after he ran the successful 1983 campaign.
When he informed Thatcher on the day of her triumph that his ex-secretary Sara Keays was pregnant, she apparently told him: “Anthony Eden leapt into bed with any good-looking woman. You can sort this out.”
However, they agreed that the secret scandal meant he could not accept any of the most high-profile roles, and instead he became secretary of state for trade and industry.
Despite Thatcher’s efforts to protect him, Parkinson was eventually forced to resign when the situation emerged publicly the following autumn.
He was later rehabilitated as energy secretary and transport secretary, but never really shook off the controversy. He quit government along with Thatcher in 1990 and entered the House of Lords two years later.
In 1997 Parkinson made a surprise comeback as Tory chairman under William Hague in the wake of the party’s electoral hammering, but only stayed in the job for a year. He retired from the Lords last September as his health deteriorated.
A family spokesman said: “Cecil passed away on 22 January after a long battle with cancer.
“We shall miss him enormously. As a family, we should like to pay tribute to him as a beloved husband to Ann and brother to Norma, and a supportive and loving father to Mary, Emma and Joanna and grandfather to their children.
“We also salute his extraordinary commitment to British public life as a member of parliament, Cabinet minister and peer – together with a distinguished career in business.
“There will be a private family funeral. The family requests that their privacy be respected in this matter. Details about a memorial service will be announced later.”
Former Cabinet minister Sir Malcolm Rifkind said Lord Parkinson could have gone on to succeed Margaret Thatcher as Conservative Party leader and prime minister if it had not been for the scandal over his affair with Ms Keays.
“Cecil Parkinson would have been the most natural candidate to succeed Margaret Thatcher because she would have had tremendous confidence in him as someone who shared her basic theme,” he told BBC News.
“In her words he was ‘one of us’. He shared her views, her thoughts, her ideas. They came from the same position on the political spectrum and therefore she was comfortable with him and had confidence in him. In addition to that, at a personal level he was able to charm her. He was a very good-looking, very handsome man. She was attracted by men who were both good-looking but also had strong principles and strong views.”
Another former Cabinet minister, Michael Portillo, said Lord Parkinson was a “very helpful right-hand man” to Mrs Thatcher and a “very well-organised” party chairman.
“He was very much of (Mrs Thatcher’s) mind. He had an easy-going charm and charisma,” he told the BBC.
“He was a very attractive man, and she liked to have around her manly men, and he absolutely suited what she was looking for.”