After graduation, in the autumn of 1970 Follett took a three-month post-graduate course in journalism and went to work as a trainee reporter in Cardiff on the South Wales Echo. After three years in Cardiff, he returned to London as a general-assignment reporter for the Evening Standard. Finding the work unchallenging, he eventually left journalism for publishing and became, by the late 1970s, deputy managing director of Everest Books. He also began writing fiction on evenings and weekends as a hobby. Success came gradually at first but the publication of Eye of the Needle in 1978 made him both wealthy and internationally famous. Each of Follett's subsequent novel has also become a best-seller, and a number have been adapted for the screen.

Follett became involved, during the late 1970s, in the activities of Britain's Labour Party. In the course of his political activities, he met the former Barbara Broer, a Labour official, who became his second wife in 1984. She was elected a Member of Parliament in 1997, representing Stevenage. She was re-elected both in 2001 and in 2005. Follett himself remains a prominent Labour supporter and fundraiser.

Follett's literary career has gone through four distinct phases.

The first, and most distinguished phase comprises Eye of the Needle and the five books that followed it. All are variations of the classic espionage thriller, pitting one or two daring, resourceful agents against a numerous and well-equipped enemy. Like the early works of Frederick Forsyth, another journalist-turned-novelist, Follett's early thrillers devote much attention to how things are done. The Key To Rebecca, for example, hinges on the workings of a particular type of secret code, the hero of Triple is a master of disguise, and clandestine radio transmitters play a major role in Eye of the Needle. All six books follow the basic conventions of the thriller genre.

The second phase of Follett's career was a conscious departure from the first: a series of four historical novels written in the late 1980s and early 1990s. The Pillars of the Earth, the first of the four, set the pattern for the three that followed. Unlike Follett's earlier thrillers, it featured a large cast, multiple plotlines, occasional outbursts of violence, and extensive use of historical background. Pillars, set mostly in medieval England, followed the building of a cathedral. Night Over Water was a Grand Hotel-style tale that took place aboard a transatlantic seaplane flying from Southampton to New York on the eve of World War II. A Dangerous Fortune revolved around family and business intrigue in a large family of financiers in Victorian-era London, and A Place Called Freedom took place in Britain's North American colonies around the time of the American Revolution.

Follett changed literary gears a third time in the late 1990s, with a pair of books set firmly in the present and using high technology as a plot device. The Hammer of Eden focused on the potential use of earthquakes as a terrorist weapon, and The Third Twin on the darker aspects of biotechnology.

Follett returned to conventional low-tech thrillers in Code to Zero, an espionage story pitting Soviet and American agents on the eve of America's first satellite launch. The World War II adventures Jackdaws and Hornet Flight put Follett firmly back where he began: writing about daring agents operating undercover behind enemy lines, charged with a mission that could change the course of the war.

Barring another radical shift in his literary output, Follett's reputation is likely to rest on his early thrillers (especially Eye of the Needle and The Key to Rebecca) and on The Pillars of the Earth, which he himself is said to regard as his finest work.