He was born in Esquesing Township (today part of Halton Hills, Ontario), the third child of Northern Irish immigrants Robert Murray Stevens and Anna Bailey McKnight. The family later moved near Kleinburg, Ontario.
He attended Weston Collegiate Institute and later, the University of Western Ontario, class of 1950. He was active in the student newspaper and the model Parliament. He entered Osgoode Hall Law School, where he met his fellow student and future wife Noreen Mary Terese Charlebois. Noreen was one of just five women in their class. They graduated in 1955 and married in 1958. From his university days until he articled, he was a part-time reporter for the Toronto Star. Stevens articled with Toronto law firm Fraser & Beatty. He later formed his own firm Stevens, Hassard & Elliot.
In 1958 his first development, The Cardiff, was under way. This was followed up with several other development projects.
In 1962 he formed York Trust and Savings Co. Former Bank of Canada governor James Coyne became chairman in 1963. Stevens had interests in several other small trust companies. Unusual for the time, his branches were located in working class areas and Loblaws stores, featuring extended service hours. York Trust grew at four times the rate of other trust companies.
By 1964 Stevens controlled 23 companies with assets of $130 million, having started in 1961 with just $215,000.
From 1963 through 1967 Stevens was embroiled in an attempt to form the first new Canadian chartered bank in fifty years, Westbank. This caused resentment in several quarters: Westerners saw it as yet another eastern-controlled firm; Conservatives were put off by the association with Coyne; and the feathers of the establishment banks were ruffled. The affair led to a falling-out with Coyne, and later with businessman Marc Bienvenu. John Diefenbaker reportedly "loathed" Stevens over the issue.
In 1968, he moved to King Township, Ontario.
He was first elected to the Canadian House of Commons in the 1972 federal election as a Progressive Conservative Member of Parliament, defeating Liberal incumbent cabinet minister John Roberts in the riding of York-Simcoe. He won again in the elections of 1972, 1974, 1979, 1980 and 1984.
Stevens ran as a candidate in the 1976 Progressive Conservative leadership convention. At the time, he had only three years parliamentary experience, but five of the other candidates had also entered parliament in 1972. He finished seventh (of eleven candidates) on the first ballot, and withdrew in favour of the eventual victor Joe Clark. This was seen as a surprising move, since Stevens was considered right-wing while Clark, a moderate, was on the party's left wing. Mulroney would "think about Steven's dramatic walk for years to come, never pretending to understand it"
Stevens had been the top official campaign spender (at $294,107) although Mulroney (who did not provide figures) is widely thought to have exceeded this
Stevens served as President of the Treasury Board in the short-lived (1979–1980) Clark government.
Stevens turned against Clark, and was an early supporter of Brian Mulroney's leadership bid which culminated in victory at the 1983 Progressive Conservative leadership convention. After the 1984 election, which resulted in a Tory landslide, Stevens became Minister of Regional Industrial Expansion
As a cabinet minister, Stevens had placed his business holdings in a blind trust. He was forced to resign from Cabinet in 1986 following allegations of conflict of interest. In December 1987, a special commission of inquiry headed by Justice William Dickens Parker ruled he had violated conflict of interest allegations on fourteen counts. David Scott (brother of Ian Scott), and Marlys Edwardh were prominent lawyers involved in the commission, which cost more than $2.9 million. Edwardh had studied search and seizure, and the Parker commission was one of the first to make extensive use of subpoena. John Sopinka represented Stevens, while Ian Binnie represented Canada. Session lasted eight months, 93 witnesses were called and nearly 14000 pages of transcripts were recorded. Scott's summation ran to 795 pages, while Sopinka's was 346 pages.
Stevens won the party nomination in his riding once again, but Prime Minister Brian Mulroney refused to sign Stevens's nomination papers, forcing the riding association to nominate another candidate. As a result of this bitter fight Stevens left Parliament in 1988.
In December 2004, Federal Court judge John O'Keefe declared null and void the findings of the Parker Inquiry. The court ruled that Parker's definition of conflict of interest exceeded that in the guidelines governing ministers in the Mulroney Cabinet, and that this step exceeded Parker's mandate. In voiding the definition of conflict of interest, the judge found that Stevens's behaviour did not violate the guidelines that governed him, since no valid guidelines existed.
Stevens returned to prominence as a bitter opponent of the merger of the Canadian Alliance and the Progressive Conservatives into the Conservative Party of Canada. Stevens backed an unsuccessful lawsuit to try to block the merger.
In 2007, following Tracy Parsons's resignation as leader of the Progressive Canadian Party, Stevens became that party's interim leader and remained in that position until his death, nine years later.