Street-Porter was born Janet Bull in Bow, daughter of an electrician and a school dinner lady. She grew up in Fulham and Perivale. Her family, she says, were poor. She went to Lady Margaret School in Parsons Green from 1958 to 1964 and then spent two years at the Architectural Association School of Architecture, where she met her first husband, photographer Tim Street-Porter.

She dropped out of college and found media work. After a brief stint at a girls' magazine called Petticoat, she joined the Daily Mail in 1969, where she became the deputy fashion editor. She became fashion editor of the Evening Standard in 1971.

When the LBC local radio station began to broadcast in 1973 Street-Porter co-presented a mid-morning show with Fleet Street columnist Paul Callan. The intention was to sharply contrast the urbane Callan and the cockney Street-Porter. Their respective accents became known to the station's studio engineers as "cut-glass" and "cut-froat". Friction between the ill-assorted pair involved constant one-upmanship that made for compelling listening, causing, it was claimed, more than one traffic accident.

In early 1975, Street-Porter was launch editor of Sell Out, an off-shoot of the London listings magazine Time Out, alongside its publisher and her second husband Tony Elliott. This was not a success.

Street-Porter went into television at LWT in 1975, first as a reporter on a series of mainly youth-oriented programmes, including The London Weekend Show (1975—79). She went on to present the late-night chat show Saturday Night People (1978—80) with Clive James and Russell Harty. She later produced Twentieth Century Box (1980—82), presented by Danny Baker.

She was editor of the innovative Channel Four Network 7 show from 1987. The same year the then BBC 2 boss Alan Yentob appointed her head of youth and entertainment features. She was responsible for the twice-weekly DEF II and commissioned Rapido, Red Dwarf and Rough Guide. Her Network 7 show was in 1988 awarded a BAFTA for its graphics.

In 1992 she provided the story for A Soap Opera, the BBC's adaptation of Heinrich August Marschner's opera Der Vampyr, which featured a new libretto by Charles Hart.

Street-Porter's approach did not endear her to critics, who objected to her diction and questioned her suitability as an influence on Britain's youth. In her final year at the BBC she became head of independent commissioning. She left the BBC for Mirror Group Newspapers in 1994 to become joint-managing director with Kelvin MacKenzie of the ill-fated L!VE TV channel. She left after four months. In 1996 Street-Porter set-up her own production company.

She has appeared on numerous reality TV shows, including Call Me a Cabbie and So You Think You Can Teach. The latter saw her trying to work as a primary school teacher. She was a contestant in the ITV series I'm a Celebrity... Get Me Out of Here!. She finished fourth.

Street-Porter conducted numerous interviews with business figures and others for Bloomberg TV.

In 2006 she appeared regularly on chef Gordon Ramsay's The F-Word, where she was the "field correspondent." In this capacity, it was her job to locate outlandish or unusual food such as crocodile and then tempt diners to have a taste. In the third series of the show she caused controversy when she attempted to serve up horse meat at Cheltenham Racecourse. She was thwarted by the police, who described the stunt as highly provocative, and she had to dish the meat out elsewhere. Ramsay himself became the target of animal rights protesters, who dumped a ton of horse manure outside his restaurant at Claridge's.

In 2007 Street-Porter starred in an ITV2 reality show called Deadline. She served as a tough-talking editor who worked with a team of celebrity "reporters" whose job it was to produce a weekly gossip magazine. The celebrities in question had to endure the Street-Porter tongue as she decided each week which of them to fire.

In 2000 Street-Porter was nominated for the "Mae West Award for the most outspoken woman in the industry" at Carlton Television's Women in Film and Television Awards. In 2008 Street-Porter was host of Celebrity Big Brother Hijack.

Her distinctive voice has made her a favourite of TV and radio impersonators. Pamela Stephenson mocked Street-Porter on Not the Nine O'Clock News (1979—82) and Kenny Everett also imitated her.

Street-Porter became editor of the Independent on Sunday in 1999. Despite derision from her critics, she took the paper's circulation up to 270,460, an increase of 11.6 per cent. In 2002 she became editor-at-large, writing a regular column.

She has written for numerous newspapers and magazines.

Following the death of Ian Tomlinson, Street-Porter dedicated her editor-at-large column in the Independent on Sunday to painting a picture of Tomlinson as a "troubled man with quite a few problems":

"Knowing that he was an alcoholic is critical to understanding his sense of disorientation and his attitude towards the police, which might on first viewing of the video footage, seem a bit stroppy."

This provoked a backlash from readers, who accused her of attempting to smear Tomlinson in order to exonerate the police. She denied that this had been her intention.

Street-Porter has also attracted criticism over an article on depression in the Daily Mail in May 2010 in which she said "Depression? It's just the new trendy illness!" People have criticised the article for being condescending to those who suffer from depression and belittling the issue.

Street-Porter was president of the Ramblers' Association for two years from 1994. She walked across Britain from Dungeness in Kent to Conway in Wales for the series Coast to Coast in 1998. She also walked from Edinburgh to London in a straight line in 1998, for a television series and her book As the Crow Flies.

[[File:CZWG house Janet Street-Porter.jpg|thumb|The Clerkenwell house commissioned by Janet Street-Porter]] In 1994, for the documentary series ''The Longest Walk'', she accompanied long-distance walker [[Ffyona Campbell]] on the last section of her round-the-world walk.

In 1987 Street-Porter commissioned a house from CZWG Architects. The building, its exterior a postmodernist mini-echo, conscious or not, of Broadcasting House, stands out among Clerkenwell's mainly Georgian houses.

In 1966 Street-Porter appeared as an extra in the nightclub scene in Blowup. In 2003 she wrote and presented a one-woman show at the Edinburgh Festival titled All the Rage. She published the autobiographical Baggage in 2004, about her childhood in working class London. Its sequel is titled Fallout. 'Life's Too F***ing Short' is a volume which presents, as she puts it, her answer to "getting what you want out of life by the most direct route."

Outspoken across many issues, Janet Street-Porter writes a regular essay in the Daily Mail. In 2009, she wrote an article entitled "why I hate Facebook", expressing a view that chat rooms were "pathetic" and warned of the risks of fraud, entrapment, grooming and that social networking was a violation of privacy.

A friend of the model Elizabeth Hurley, Street-Porter danced with Hurley and six others at Indian-style celebrations the night before the Hurley's marriage to Arun Nayar in 2007.

Street-Porter has been married four times. Her first marriage, to photographer Tim Street-Porter ended in 1974 after her relationship with Tony Elliott, whom she was married to until 1977. She later married Canadian film-maker Frank Cvitanovich and then David Sorkin. She is now in a relationship with restaurateur Peter Spanton.

She lives in Nidderdale, Yorkshire, Kent and London.