Monty Don was born in West Berlin to British parents Denis Thomas Keiller Don, a career soldier posted in Germany, and Janet Montagu (née Wyatt). Both of his paternal grandparents were Scottish, through whom he is descended from botanist George Don and the Keiller family of Dundee, inventors of a brand of marmalade in 1797. On his maternal side, he is descended from the Wyatts, who were a prominent dynasty of architects. Both parents died in the 1980s. Don has a twin sister, an elder brother David, and two other siblings. His twin suffered a broken neck and blindness after a car crash, at the age of 19. Don describes his parents as being "very strict".
Don was educated at three independent schools: Quidhampton School in Basingstoke, Hampshire, Bigshotte School in Wokingham, Berkshire, and at Malvern College in Malvern, Worcestershire, a college he hated. He then attended a state comprehensive school, the Vyne School, in Hampshire. He failed his A levels and while studying for re-takes at night school, worked on a building site and a pig farm by day. During his childhood he had become an avid gardener and farmer. He was determined to go to Cambridge out of "sheer bloody-mindedness", attending Magdalene College, where he read English and met his future wife Sarah (née Erskine). He was a Cambridge Half Blue for boxing.
In the 1980s Don and his wife formed a successful company that made and sold costume jewellery under the name Monty Don Jewellery. The collapse of the company in the early 1990s prompted him to embark on a career in writing and broadcasting. He has written about the rise and collapse of their business in The Jewel Garden, an autobiographical book written with his wife. Reflecting on the experience, he writes "We were lambs to the slaughter and we lost everything, [...] we lost our house, our business. We sold every stick of furniture we had at Leominster market". He was unemployed from 1991 to 1993.
Don's first TV work came as the presenter of a gardening segment on breakfast show This Morning. He featured as a guest presenter for the BBC's Holiday programme. From 1994 to 1995 he appeared from time to time as one of the presenters for the weekly science programme Tomorrow's World on BBC One. He went on to present several Channel 4 land and gardening series: Don Roaming, Fork to Fork, Real Gardens and Lost Gardens, and wrote a regular weekly gardening column for The Observer between February 1994 and May 2006 and for the Daily Mail and Mail Online since 2004.
Don had never received formal training as a gardener. He commented, "I was – am – an amateur gardener and a professional writer. My only authority came from a lifetime of gardening and a passion amounting to an obsession for my own garden."
He is a keen proponent of organic gardening and the practice of organic techniques often features in his published and broadcast work. The organic approach is most prominent in his 2003 book The Complete Gardener. This has led him into some controversy with those advocating non-organic techniques, with some criticising his position of influence presenting Gardeners' World and exclusion of non-organic solutions to pests and diseases in the garden.
Don was the main presenter on BBC Two's Gardeners' World from 2003 to 2008 succeeding Alan Titchmarsh. He was the first self-taught horticulturist presenter in the show's 36-year history, stepping down only after suffering a minor stroke. After viewing figures for Gardeners' World fell below two million for the first time in 2009, in January 2010, changes were announced to the programme in an attempt to entice viewers back. In December 2010, it was announced that Don would be returning to the programme as lead presenter for the 2011 series, replacing Toby Buckland. Reaction to the announcement was divided on the programme's blog. Since March 2011 he has been presenting the programme from his own garden (called Longmeadow for the purpose of television) in Herefordshire. He is frequently seen with his Golden Retriever Nigel, who has his own Twitter account. In 2016  Don also introduced viewers of Gardeners' World to a new Golden Retriever called Nellie (born 22 September 2015) who also has her own Twitter account.
Don featured in the BBC programme and book, Growing out of Trouble, in which several heroin addicts manage a 6-acre (2.4 ha) Herefordshire smallholding in an attempt at rehabilitation. He also presented Around the World in 80 Gardens (BBC Two 27 January – 30 March 2008) and in December 2008, narrated a programme about the cork oak forests of Portugal, for the BBC's natural history series Natural World. He presented My Dream Farm, a series which helped people learn to become successful smallholders (Channel 4, January 2010) and Mastercrafts, a six-part series for BBC Two, which celebrated six traditional British crafts. He has twice been a panellist on the BBC's Question Time (February 2009 and March 2010) and his family history was the subject of the fourth programme in the seventh series of the BBC genealogy programme Who Do You Think You Are? (August 2010). In April 2011 Don presented Italian Gardens, a four-part BBC2 series which was accompanied by the publication of a book. In February 2013, a companion series, Monty Don's French Gardens, was broadcast.
In 2013 Don presented an episode of Great British Garden Revival. In 2015 he presented The Secret History of the British Garden, a BBC Two series, in four parts, charting the development of British gardens from the 17th to the 20th century. Since 2014, Don has presented three series of Big Dreams, Small Spaces on BBC Two, where he helps amateur gardeners in creating their own 'dream spaces' on a domestic scale.
Don's sartorial style in the garden has been the subject of some critical attention, with Richard D. North commenting, in 2013:
At home and abroad, Monty Don is the paysan manqué. Where an arts presenter might eschew the little black Armani suit and the dazzling white shirt for the crumpled linen, Don’s gear retreats into the manly rumpledon of a workman’s cotton drill. He is not quite the Mr McGregor of the Potter books: real-life ancient gardeners wore mighty cords and moleskins, tweeds and flannels – and sacks if the weather was bad enough. The Don affectation is one tad more painterly than that.
... I guess that this is where we come up against the row within Monty Don, between the lightly earthy garden enthusiast and the grimmer unworldly hippy moralist. Well, we all have an inner cheerfully accepting Cavalier, and it does battle with our gloomier Roundhead.
In 2005 Don himself dedicated a whole column to this subject, commenting:
"I get lots of emails, lots of letters. A few are crazed, quite a few astonishingly demanding...quite a surprisingly large chunk of letters and emails are about one specific topic that is at first appearance only tangentially about gardening. These are the ones asking me about the clothes that I garden in."
In January 2018 he presented the two-part BBC series Paradise Gardens, travelling across the Islamic world and beyond in search of paradise gardens and considering their place in the Quran. In February 2019 a similar two-part BBC series Japanese Gardens gave similar treatment to gardens in Japan. An American Gardens series was shown in 2020.
Monty and Sarah married in 1983 and have three children. The couple lived in Islington, north London, while Don pursued postgraduate study at the London School of Economics, worked as a waiter at Joe Allen restaurant in Covent Garden and later as a dustman, and completed two unpublished novels (The Clematis Affair and An Afternoon in Padua), described as "excruciatingly bad" in The Jewel Garden. Meanwhile, Sarah trained as a jeweller. The couple then lived in Ufton Road, De Beauvoir where they made their first garden. In 1989 Don moved to The Hanburies, a large country house in Herefordshire. The making of the gardens there and the subsequent loss of the house in the aftermath of the crash of their business was the subject of his first published book The Prickotty Bush. He has lived at Ivington, Herefordshire, England since 1991. He has three dogs: Nigel (a Golden Retriever), Nellie (a Golden retriever new for 2016) and Brenda (a Miniature Dachshund). The coppice at Longmeadow holds the graves of Dons' other dogs, marked by headstones: Beaufort, Red, Poppy and Barry, as well as two cats Stimpy and Blue.
Don has written of his struggle with depression since the age of 25 and Seasonal Affective Disorder. He describes in his memoir "great spans of muddy time" in which there is nothing but depression. He noted "'Earth heals me better than any medicine". He has had cognitive behavioural therapy and took Prozac before favouring a lightbox, now a recognised aid for Seasonal Affective Disorder sufferers. He had peritonitis in 2007 and a minor stroke in 2008. Shortly before he fell ill with his stroke, Don had launched the Monty Don Project, a charity to help persistent offenders and drug addicts heal themselves by working the land.
In July 2006 he appeared on BBC Radio 4's Desert Island Discs, choosing an eclectic mix of pop and classical records; the Beatles' "A Hard Day's Night" was his favourite disc, his book choice was Collected Poems by Henry Vaughan and his luxury item the painting by Rembrandt.
In May 2016 Don revealed that years of gardening had left him with "dodgy knees", from which he was "almost constantly in pain". But he dismissed any suggestion of replacement joints, saying: "Listen, when you get to 60, you ache. Just take it."