Born Isabella Delves Broughton in Marylebone, London, England, she was the eldest child of Major Sir Evelyn Delves Broughton, a military officer, and his second wife, Helen Mary Shore, a barrister. Sir Evelyn was the only son of Jock Delves Broughton; his sister, Rosamond, married Simon Fraser, 15th Lord Lovat in 1938.
Blow had two sisters, Julia and Lavinia, and a brother, John, who drowned in the family's swimming pool at the age of two. In 1972, when she was 14, her parents separated and her mother left the household, bidding each daughter farewell with a handshake. Her parents divorced two years later. Isabella did not get along with her father, who bequeathed her only £5,000 from his estate, which was worth more than one million pounds. Blow often said her fondest memory was trying on her mother's pink hat, a recollection that she explained led to her career in fashion.
I've done the most peculiar jobs. I was working in a scone shop for years, selling apricot-studded scones. I was a cleaner in London for two years. I wore a handkerchief with knots on the side, and my cousin saw me in the post office and said, What are you doing? I said, What do you think I look like I'm doing? I'm a cleaner!
Blow moved to New York City in 1979 to study Ancient Chinese Art at Columbia University and shared a flat with the actress Catherine Oxenberg. A year later, she left the Art History programme at Columbia, moved to Texas, and worked for Guy Laroche. In 1981, she married her first husband, Nicholas Taylor (whom she divorced in 1983), and was introduced to the fashion director of the U.S. edition of Vogue, Anna Wintour. She was hired initially as Wintour's assistant, but it was not long before she was assisting Andre Leon Talley, now U.S. Vogue's editor-at-large. While working in New York, she befriended Andy Warhol and Jean-Michel Basquiat.
In 1989, Blow married her second husband, art dealer Detmar Hamilton Blow, in Gloucester Cathedral; he is a grandson (and namesake) of the early 20th-century society architect Detmar Blow. Philip Treacy designed the bride's wedding headdress and a now-famous fashion relationship was forged. Realizing Treacy's talent, Blow established Treacy in her London flat, where he worked on his collections. She soon began wearing Treacy's hats, making them a signature part of her flamboyant style. In a 2002 interview with Tamsin Blanchard, Blow declared that she wore extravagant hats for a practical reason:
...to keep everyone away from me. They say, Oh, can I kiss you? I say, No, thank you very much. That's why I've worn the hat. Goodbye. I don't want to be kissed by all and sundry. I want to be kissed by the people I love.
In 1993, Blow worked with the photographer Steven Meisel producing the Babes in London shoot featuring Plum Sykes, Bella Freud, and Honor Fraser. Blow had a natural sense of style and a good feeling for future fashion directions. She discovered Alexander McQueen and purchased his entire graduate collection for ₤5,000, paying it off in weekly ₤100 installments. Spotting Sophie Dahl, Blow described Dahl as "a blow up doll with brains", and launched the model's career.
Blow was the fashion director of Tatler and consulted for DuPont Lycra, Lacoste, and Swarovski. In 2002, she became the subject of an exhibition entitled When Philip met Isabella, featuring sketches and photographs of her wearing Treacy's hat designs.
In 2004, she had a brief acting cameo in the film The Life Aquatic with Steve Zissou.
In 2005, Blow starred in a project by artist Matthieu Laurette, commissioned and produced by Frieze Projects 2005 and entitled "What Do They Wear at Frieze Art Fair?" It consisted of daily guided tours of Frieze Art Fair led by Blow and fellow international fashion experts Peter Saville, Kira Joliffe, and Bay Garnett.
Shortly before her death, she was the creative director and stylist of a series of books about beauty in the Arab world; the books were being produced by Kuwaiti fashion entrepreneur Sheikh Majed al-Sabah. Blow was dismissed from the project for unknown reasons and attempted suicide.
Toward the end of her life, Blow had become seriously depressed and was reportedly anguished over her inability to "find a home in a world she influenced". Daphne Guinness, a friend of Blow's stated, "She was upset that Alexander McQueen didn't take her along when he sold his brand to Gucci. Once the deals started happening, she fell by the wayside. Everybody else got contracts, and she got a free dress". According to a 2002 interview with Tamsin Blanchard, it was Blow who brokered the deal in which Gucci purchased McQueen's label.
Other pressures included money problems (Blow was disinherited by her father in 1994) and infertility. In an effort to have a child, Blow and her husband had unsuccessfully tried in vitro fertilization eight times. She later stated, "We were like a pair of exotic fruits that could not breed when placed together."
In 2004, Isabella and Detmar Blow separated. Detmar Blow went on to have an affair with Stephanie Theobald, the society editor of British Harper's Bazaar, while his estranged wife entered into a liaison with a gondolier she met in Venice. During the couple's separation, Blow was diagnosed with a bipolar disorder and began undergoing electroshock therapy. For a time, the treatments appeared to be helpful. After an eighteen month separation, Isabella and Detmar Blow were reconciled. Soon after, she was diagnosed with ovarian cancer.
Depressed over her waning celebrity status and her cancer diagnosis, Blow began telling friends that she was suicidal. In 2006, Blow attempted suicide with an overdose of sleeping pills. Later that year, Blow again attempted suicide by jumping from the Hammersmith Flyover, which resulted in her breaking both ankles.
In 2007, Blow made several more suicide attempts by driving her car into the rear of a truck, attempting to obtain horse tranquilizers, trying to drown herself in a lake and by overdosing while on a beach in India.
On May 6, 2007, during a weekend house party at Hilles, where the guests included Treacy and his life partner, Stefan Bartlett, Blow announced that she was going shopping. Instead, she was later discovered collapsed on a bathroom floor by her sister Lavinia and was taken to Gloucestershire Royal Hospital, where Blow told the doctor she had drunk the weedkiller Paraquat. Blow died at the hospital the following day.
Blow's death was initially reported as being caused by ovarian cancer; however, a coroner later ruled the death a suicide. At the inquest, Blow's sister, Lavinia Verney, stated that after she discovered her sister had ingested the poison, Blow had told her, "I'm worried that I haven't taken enough."
After her death, Detmar Blow confirmed that his wife suffered from depression and that she had once declared, "I'm fighting depression and I can't beat it".
Her funeral was held at Gloucester Cathedral on May 15, 2007. Her coffin, made of willow, was surmounted by one of her Philip Treacy hats instead of a floral tribute, and her pallbearers included her godson Otis Ferry, a son of the rock star Bryan Ferry. (In 2010, Bryan Ferry dedicated his Olympia album in memoriam Isabella Blow and David Williams.) Actor Rupert Everett and actress Joan Collins delivered eulogies. A memorial service was held in the Guards Chapel in London on September 18, 2007, where Anna Wintour and Geordie Greig spoke. Wintour's eulogy and part of the memorial service can be seen in DVD disc 2 of The September Issue.