Lenon began his career with a year at Sherborne School in Dorset,[7] then moved on to Eton College, an independent school in Berkshire, where he taught for twelve years. He was in the state sector for a short while at Holland Park School in west London. He became deputy headmaster of Highgate School and then headmaster of the Trinity School of John Whitgift in Croydon, south London. He was appointed as headmaster of Harrow in 1999 and during his tenure the school increased in both popularity and academic results. He was appointed Chairman of the Independent Schools Council in 2011 to oversee lobbying, research, the press and legal advice for the 1250 schools in ISC.
Lenon is often quoted in the media about his views, complaining of what he regards as the 'dumbing down' of educational standards since the demise of O-Levels, and in early 2010 was attacked by the then Schools Minister in the Labour Government, Vernon Coaker.[8][9]
Lenon has appeared in newspapers and on the television a number of times in the past decade, mostly speaking about educational reforms. He appeared in the ITV documentary series Harrow: The School on the Hill in May 2001, on BBC 1's Celebrity Masterchef programme in August 2010, and on the BBC 2 documentary programme Britain's Youngest Boarders in September 2010. He has also appeared on BBC 2's Newsnight programme.
Lenon has published a number of successful textbooks for schools including Techniques and Fieldwork in Geography (1983),[ISBN missing] London (1988), London in the 1990s (1993), The United Kingdom (1995), Fieldwork Techniques and Projects in Geography (1996),[ISBN missing] Geography Fieldwork and Skills (2015).[ISBN missing]
In 2017 he published a review of the national education system Much Promise: successful schools in England ISBN 978-1911382232
In 2018 he published 'Other Peoples Children', an account of what happens to those in England's bottom 50% academically and the state of vocational education in the country.