Sewell was born and brought up in Kensington, London, and educated at the independent Haberdashers' Aske's Boys' School and then Courtauld Institute of Art, where he was tutored by Anthony Blunt and became his close friend. Sewell graduated in 1957 and worked at Christie's auction house, specialising in Old Master paintings and drawings. After leaving Christie's he became an art dealer. He did his National Service in the Royal Army Service Corps, in which he was commissioned. He has since been a regular commentator on the ITV series Bad Lad's Army, offering insight into 1950s military life.

In 1984 he became art critic of the Evening Standard (replacing avant-garde critic Richard Cork). He won press awards including Critic of the Year in 1988, Arts Journalist of the Year in 1994, the Hawthornden Prize for Art Criticism in 1995 and the Foreign Press Award (Arts) in 2000. In April 2003 he was awarded "The George Orwell" prize for his political/current affairs column in the Evening Standard. In criticisms of the Tate Gallery's art, he coined the phrase, the "Serota Tendency", after its director Nicholas Serota. It was not until the late 1990s that he became a household figure through television, though he was on BBC Radio 4 before then.

Sewell is noted for formal, old-fashioned diction and anti-populist sentiments. He offended people in Gateshead by claiming an exhibition was too important to be held only at the town's Baltic Centre for Contemporary Art and should be shown to "more sophisticated" audiences in London; he has also disparaged Liverpool as a cultural city.

In 1994, 35 art world signatories wrote a letter to the Evening Standard attacking Sewell for "homophobia", "misogyny", "demagogy", "hypocrisy", "artistic prejudice", "formulaic insults and predictable scurrility". Signatories included Karsten Schubert, Maureen Paley, Michael Craig-Martin, Angela and Matthew Flowers, Professor Christopher Frayling, Rene Gimpel, Susan Hiller, John Hoyland, Sarah Kent, Nicholas Logsdail, George Melly, Sandy Nairne, Sir Eduardo Paolozzi, Bridget Riley, Michelle Roberts, Richard Shone, Marina Warner, Natalie Wheen, and Rachel Whiteread. He responded with comments on many of the signatories, saying that Paley was "The curatrix of innumerable silly little Arts Council exhibitions", and that Whiteread was "Mortified by my dismissal of her work for the Turner Prize". A letter supporting Sewell from 20 other art world signatories accused the writers of attempted censorship to promote "a relentless programme of neo-conceptual art in all the main London venues".

Sewell's attitude to female artists has been controversial. In July 2008 he was quoted in The Independent as saying:

"The art market is not sexist. The likes of Bridget Riley and Louise Bourgeois are of the second and third rank. There has never been a first-rank woman artist. Only men are capable of aesthetic greatness. Women make up 50 per cent or more of classes at art school. Yet they fade away in their late 20s or 30s. Maybe it's something to do with bearing children."

Sewell does not hold his tongue regarding his opinions, and has frequently insulted the general public for their views on art. Consequently, he is more known for controversy than art criticism among many. He has issued quotes such as the following regarding public praise for the work of Banksy in Bristol:

"The public doesn't know good from bad. For this city to be guided by the opinion of people who don't know anything about art is lunacy. It doesn't matter if they [the public] like it."

He went on to assert that Banksy himself "should have been put down at birth."Clive Anderson has described him as "a man intent on keeping his Christmas card list nice and short."

Sewell is also known for his disdain for Damien Hirst, describing him as "fucking dreadful."

In 2003, Sewell made a pilgrimage to Santiago de Compostela, as a documentary called The Naked Pilgrim, produced by Wag TV for Channel 5. Although he has not practised for decades, Sewell considers himself a Roman Catholic, prompting an emotional response to the faith of pilgrims at Lourdes. The series attracted large audiences and won the Sandford St. Martin Trust award for Best Religious Programme. Following The Naked Pilgrim Sewell presented on two more series for Channel 5: Brian Sewell's Phantoms & Shadows: 100 Years of Rolls-Royce in 2004 and Brian Sewell's Grand Tour, 2006. Sewell also appeared as a guest film reviewer on Channel 5's Movie Lounge, where he frequently savaged films.

In Dirty Dalí: A Private View on Channel 4 on 3 June 2007, Sewell described his acquaintance with Salvador Dalí in the late 1960s, which included lying in the foetal position without trousers in the armpit of a figure of Christ and masturbating for Dalí, who pretended to take photos while fumbling in his trousers.

Sewell has appeared twice as panellist on the BBC's news quiz Have I Got News For You? and tried to teach cricketer Phil Tufnell about art in ITV's Don't Call Me Stupid.

He acted as Big Brother during 2008's Celebrity Hijack.

He was the voice of Sir Kiftsgate in an episode of the children's cartoon The Big Knights.

Sewell has a show on Voom HD Network's Art Channel: Gallery HD called Brian Sewell's Grand Tour, in which he tours beautiful cities (primarily in Italy) visiting museums, towns, churches, historic sights, public squares, monuments, profound architectural spots and meeting a local to discuss culture and art. Sewell reflects the 18th century, giving the perspective of what it would have been like as a 'Grand Tourist'. Then he elaborates on what has become of these sights and lost throughout history.

In a 2009 BBC documentary about the so-called North-South Divide, presented by ex-Deputy Prime Minister John Prescott, Sewell caused controversy by declaring that the solution to the divide was to send a pox or a plague upon the North so they can all just die quietly.

Television credits

Sewell is a museum adviser in South Africa, Germany and the United States. He is also patron of the British charity NORM-UK which raises awareness of circumcision and other forms of surgical alteration of the genitals. Sewell has also provided voice-overs for a variety of television commercials including the Victorian and Albert Museum and Feta cheese.

Brian Sewell is also a noted aficionado of classic automobiles, a fan of stock car racing and over several decades has written extensively about cars, classic and contemporary, in the Evening Standard and elsewhere. In both his TV series, on the pilgrimage to Santiago and the Grand Tour (supra), he drove his venerable Mercedes-Benz 560 SEC coupe.

In a television programme broadcast on Channel 4 on 24 July 2007, marking the 40th anniversary of the passing of the Sexual Offences Act 1967 which partially decriminalised homosexuality in England and Wales, Sewell said, "I never came out... but I have slowly emerged". Sewell is bisexual but rather chastises himself for this, describing his attraction to men 'as an "affliction" and a "disability" and [told] readers, "no homosexual has ever chosen this sexual compulsion"'. In the first episode of The Naked Pilgrim Sewell alluded to the loss of his virginity at the hands of a 60 year old French woman "who knew what she was doing and was determined."; Sewell was 20 at the time.

In an interview for That's Mad website, Sewell revealed that he suffers from depression, and as a cure usually just sleeps for a long time during the more severe phases. That's Mad: Is Brian Sewell the Best Thing on Television?

John Humphrys, in his book Lost for Words, writes "They (people who deliberately speak 'poshly') try to speak like the Queen or even Brian Sewell, the only man I have ever met who makes the Queen sound common."