Monckton joined the Yorkshire Post in 1974 at the age of 22, where he worked as a reporter and leader-writer. From 1977 to 1978, he worked at Conservative Central Office as a press officer, becoming the editor of the Catholic newspaper The Universe in 1979, then managing editor of The Sunday Telegraph Magazine in 1981. He joined the London Evening Standard newspaper as a leader-writer in 1982. After a hiatus in his career as a journalist Monckton became assistant editor of the newly established, and now defunct, tabloid newspaper Today in 1986. He was a consulting editor for the Evening Standard from 1987 to 1992 and was its chief leader-writer from 1990 to 1992.

He was Special Advisor to British Prime Minister Margaret Thatcher from 1982 to 1986, and he has served as Secretary for the Centre for Policy Studies's economic, forward strategy, health and employment study groups; Deputy Leader of the UK Independence Party (UKIP); and the UKIP's President in Scotland.

On leaving 10 Downing Street, he established a successful specialist consultancy company, giving technical advice to corporations and governments. The first of his two articles on global warming in The Sunday Telegraph in November 2006 crashed its website after attracting 127,000 hits within two hours of publication. A speech by Lord Monckton to 1,000 citizens of St. Paul, Minnesota in October 2009, in which he drew public attention to a then little-known draft plan by the U.N. to establish an unelected world government at the (now-failed) climate summit at Copenhagen in December 2009, received 1,000,000 YouTube hits in a week—thought to be the fastest-ever YouTube platinum for a political speech. Some five million have now seen the presentation on various websites.

His lecture to undergraduates at the Cambridge Union Society on climate change was released as a feature-length movie, Apocalypse? NO! He triumphed in debate at St. Andrews University, where undergraduates voted against climate alarm for the first time at any British university, and at the Oxford Union, where undergraduates voted against climate alarm for the first time at any English university.

Monckton has authored numerous papers on the climate issue both for the layman and scientific community. He established in a paper for the World Federation of Scientists that CO2 has a social benefit, not a social cost. He was also a co-author of the paper that showed the claim of “97% scientific consensus” about climate change to be false (the true figure is 0.35). His scholarly articles on climate issues have appeared in numerous books plus such journals as Science Bulletin, Energy and Environment, Journal of the Marine Navigation Industry, UK Quarterly Economic Bulletin, Physics and Society, Science & Education, and AIG News.

For his work on the climate, Lord Monckton, who was Nerenberg Lecturer in Mathematics at the University of Western Ontario in 2013, has been presented with numerous honors, including the Meese-Noble Award for Freedom, the Valiant-for-Truth Award of the Committee for a Constructive Tomorrow, the Santhigiri Ashram Award, and the Intelligence Medal of the Army of Colombia.

In 1999, Monckton created and published the Eternity puzzle, a geometric puzzle that involved tiling a dodecagon with 209 irregularly shaped polygons called "polydrafters". A £1 million prize was won after 18 months by two Cambridge mathematicians. By that time, 500,000 puzzles had been sold. Monckton launched the Eternity II puzzle in 2007, but, after the four-year prize period, no winner came forward to claim the $2 million prize.

Monckton is a liveryman of the Worshipful Company of Broderers, an Officer of the Order of St John of Jerusalem, a Knight of Honour and Devotion of the Sovereign Military Order of Malta, and a member of the Roman Catholic Mass Media Commission. He is also a qualified Day Skipper with the Royal Yachting Association, and has been a trustee of the Hales Trophy for the Blue Riband of the Atlantic since 1986.