Brightman is the eldest of six children of businessman Grenville Geoffrey Brightman (1934–1992) and Paula (Hall) Brightman. Her younger siblings are Nicola, Claudia, Jay, Joel, and Amelia (aka Violet). She was brought up in Little Gaddesden near Berkhamsted, Hertfordshire, England. At the age of three she began taking dance and piano classes and went on to perform in local festivals and competitions. At age 11, she successfully auditioned for the Tring Park School for the Performing Arts, a school specialising in performing arts.

In 1973, at the age of 13, Brightman made her theatrical debut in the musical I and Albert at the Piccadilly Theatre, London, playing one of Queen Victoria's daughters (Victoria). In 1976 she was recruited into Arlene Phillips' troupe Hot Gossip in 1977. The group had a disco hit in 1978 with "I Lost My Heart to a Starship Trooper", which sold half a million and reached number six on the UK charts. She was also briefly with Pan's People after they parted with their host show Top of the Pops in 1976. Brightman, now solo, released more disco singles under her own label, Whisper Records, such as "Not Having That!" and a cover of the song "My Boyfriend's Back". In 1979, Brightman appeared on the Soundtrack of the film "The World Is Full of Married Men" and sang the song "Madam Hyde".

In 1981, Brightman auditioned for the new musical Cats, by Composer Andrew Lloyd Webber, and was cast as Jemima. After a year in Cats, Brightman took over from Bonnie Langford as Kate in The Pirates of Penzance at the Drury Lane Theatre, London, and appeared as Tara Treetops in Masquerade, a musical based on Kit Williams's book of the same title. In that year she left to play the title role in Charles Strouse's children's opera, Nightingale.

At age 18, in 1979, Brightman married Andrew Graham-Stewart, who then managed the German band Tangerine Dream. She later met Andrew Lloyd Webber when she performed in Cats. In 1983, Brightman divorced Graham-Stewart and later that same year, Lloyd Webber divorced his first wife, Sarah Hugill, with whom he had two children. Lloyd Webber and Brightman married on 22 March 1984 and their relationship quickly became the subject of intense media and tabloid scrutiny until their divorce in 1990. Brightman acknowledged the marriage in a 1999 interview as a "difficult time" but also one of much creative output. They are currently on friendly terms; in 2006, at the 20th London anniversary of The Phantom of the Opera, Lloyd Webber called Brightman a "wonderful woman" and "absolutely beloved mentor", and she performed at the 25th anniversary of the play in 2011. He appeared as a special guest in her 1997 concert at the Royal Albert Hall in London.

In 1985, Brightman's recording of "Pie Jesu" was a strong commercial success, selling 25,000 copies on the first day of release and peaking at number 3, despite the lyrics being in Latin. With classical music permeating the Lloyd Webber household (Brightman was in heavy operatic training at the time), Lloyd Webber was moved to write the Requiem Mass as a tribute to his father. Its Manhattan premiere, starring Plácido Domingo and Brightman, was filmed by the BBC and PBS for later broadcast. The LP eventually became UK's top selling classical album of the year and earned Brightman a Grammy nomination as Best New Classical Artist."

Her original London Cast Album of the musical was released in CD format in 1987 and sold 40 million copies worldwide, making it the biggest-selling cast album of all time.

After leaving Phantom, she performed in a tour of Lloyd Webber's music throughout England, Canada and the United States, and performed Requiem in the Soviet Union. Studio recordings from this time include the single "Anything But Lonely" from Aspects of Love and two solo albums: the 1988 album The Trees They Grow So High, a collection of folk songs accompanied by piano, and the 1989 album The Songs That Got Away, a compilation of obscure musical theatre songs from shows by such composers as Irving Berlin and Stephen Sondheim. Brightman also sang the song "Make Believe" during the credits of the children's film Grandpa; Howard Blake composed the music and wrote the lyrics.

She was a subject of the television programme This Is Your Life in 1989 when she was surprised by Michael Aspel.

By 1990, Brightman and Lloyd Webber had separated. After their highly publicized divorce, Brightman played the lead in Lloyd Webber's Aspects in London opposite Michael Praed, before transferring to Broadway. Her subsequent solo album, As I Came of Age, was an eclectic collection of folk-rock and musical theatre songs that Brightman herself chose.

Brightman has suffered several personal crises. In February 1992, her 57-year-old father committed suicide by asphyxiation in his car in Hertfordshire after divorce and financial problems.

Brightman and Peterson's second collaboration yielded the pop rock album, Fly (1995). The album catapulted Brightman to fame across Europe, and she unveiled its hit track "A Question of Honor"—a mélange of electronic, rock, classical strings and excerpts from the Alfredo Catalani opera La Wally. The song and the video were introduced at the World Boxing Championship match between Germany's Henry Maske and Graciano Rocchigiani.

Brightman's 1996 duet with the Italian tenor Andrea Bocelli, "Time to Say Goodbye", topped charts all over Europe and became the highest and fastest selling single of all time in Germany, where it stayed at the top of the charts for 14 consecutive weeks and sold over 3 million copies. It subsequently became an international success selling 12 million copies worldwide, making it one of the best-selling singles of all time. She has now collected over 180 gold and platinum sales awards in 38 different countries. In 2010 she was named by Billboard the 5th most influential and best-selling classical Artist of the 2000s decade in the US and according to Nielsen SoundScan, she has sold 6.5 million albums in the country.

In 1997, Brightman released the album Timeless/Time to Say Goodbye. It remains as Brightman's biggest-selling album. It went gold, platinum and/or multi-platinum in 21 countries, selling over 1.4 million copies in the U.S. alone, and topped the Billboard Top Classical Crossover Albums chart in the U.S. for 35 weeks. The lead single from the album, "Time to Say Goodbye", was the second song that Brightman debuted at the World Boxing Championship in Germany. This duet with tenor Andrea Bocelli became an international hit and sold more than 3 million copies in Germany alone, became Germany's best-selling single, and was successful in numerous other countries; the album eventually sold over 12 million copies worldwide.

With the success of Timeless, Brightman released her next album, Eden in 1998. She hand-selected each song and convinced the Academy Award-winning Italian Composer Ennio Morricone to let her set lyrics to one of his film compositions, "Gabriel's Oboe" from the film The Mission resulting in the now classic piece "Nella Fantasia". The album, unlike Time to Say Goodbye, incorporated more pop music elements. Reviews were mixed – LAUNCHcast deemed Eden "deliriously sappy", while Allmusic called Eden "a winning combination". Eden reached No. 1 on the US Billboard Top Classical Crossover Albums chart and No. 65 on the Billboard 200 chart and was certified Gold in the United States.

Brightman's music is generally Classified as classical crossover. According to Manhattan Records GM Ian Ralfini, she is largely responsible for the popularity of the genre. In a 2000 interview with People, Brightman dismissed the classical crossover label as "horrible" but stated she understood people's need to categorise music. Her personal influences include 1960s and 1970s Musicians and artists such as David Bowie and Pink Floyd, and she incorporates aspects of genres from pop/rock to classical. Her work has also been compared to that of Madonna, Cher and Celine Dion. The material on her albums ranges from versions of opera arias from composers such as Puccini (on Harem, Eden, and Timeless), to pop songs by artists such as Kansas ("Dust in the Wind" on Eden), Dido ("Here with Me" on La Luna), and Procol Harum ("A Whiter Shade of Pale" on La Luna). She sings in many languages which are English, Spanish, French, Latin, German, Italian, Russian, Hindi, Mandarin Chinese and Japanese.

In 2001, Brightman released Classics, an anthology including highlights from three of Brightman's chart-topping releases along with seven new tracks; this was released worldwide except Europe. In the U.S. the album peaked at No. 66 on the Billboard 200 chart and went Gold. In Canada it peaked at No. 9 and was certified Platinum; and in Japan, Classics became Brightman's most successful release at the time with 300,000 units sold and reaching Platinum status.

Her 2003 album Harem represented another departure: a Middle Eastern-themed album influenced by dance music. On Harem, Brightman collaborated with artists such as Ofra Haza and Iraqi singer Kazem al-Saher. Nigel Kennedy contributed violin tracks to the songs "Free" and "The War is Over" and Jaz Coleman contributed arrangements.

In 2004 the Harem World Tour grossed $60 million and sold 700,000 tickets, $15 million and 225,000 sales of which came from the North American leg, although with ticket prices raised 30% from previous tours, average sales per venue were up 65%. In North America, Harem tour promoters Clear Channel Entertainment (now Live Nation) took the unusual step of advertising to theatre subscribers, in an effort to reach fans of Brightman's Broadway performances, and also sold VIP tickets, at $750 each, that included in-stage seating during the concert and a backstage pass. Tour reviews were mixed: one critic from the New York Times called the La Luna tour "not so much Divine but post-human" and "unintentionally disturbing: a beautiful argument of emptiness." In contrast, a reviewer from the Boston Globe deemed the Harem tour "unique, compelling" and "charmingly effective."

Brightman released a DVD collection of her music videos on 3 October 2006 under the title of Diva: The Video Collection. The Singles Collection is the accompanying CD, released on the same date. The album marked the first time Brightman released a greatest hits album in the United States; it reached No. 1 on the Billboard Top Classical Crossover Albums chart. In Japan, the album debuted and peaked at No. 2 with 77,000 copies sold on its first week of release, and became Japan's biggest-selling classical album of 2007. By 2008, the album achieved a Double-Platinum certification. With Diva, Brightman was also South Korea's best-selling international Artist of 2010 as the album topped the international charts all throughout the year. Although the original 2006 release experienced limited success, the album charted in 2009 and 2010 when Brightman toured South Korea with the Symphony World Tour and Sarah Brightman in Concert With Orchestra. Diva was certified Quintuple Platinum and its digital single, "Nella Fantasia" sold over 2 million units. Other releases in Europe were The Very Best of 1990–2000 and Classics: The Best of Sarah Brightman.

In the United Kingdom, On May 2007, Brightman sang along Soprano Lesley Garrett at Wembley Stadium the anthem "Abide With Me" before the FA Cup final between Chelsea and Manchester United, and participated as well in a dinner held at the Mansion House, where the Prince of Wales highlighted the urgent need for action to halt tropical deforestation. Brightman sang at the event to engage the financial community in the task of finding a solution to the Problem of making rainforests "worth more alive than dead". The music performed was Nella Fantasia and was further declared hymn to the rainforests.

In response to persistent calls for a global release of the Symphony: Live in Vienna concert, EMI Music launched worldwide the PBS special which featured Brightman's performance at Vienna's St. Stephen's Cathedral on 16 January 2008, in both audio and visual formats. The Symphony—Live in Vienna television special debuted on PBS in March 2008 during the network's spring pledge drive and aired throughout the month. Symphony: Live in Vienna was listed as the thirteenth best-selling album of the year in Mexico.

Brightman performed a concert tour called Sarah Brightman in Concert during October 2009. It covered South America with 13 performances in Venezuela, Colombia, Peru, Chile, Argentina and Brazil. The last venue of the tour, "The Concert of the Pyramid" featured Brightman performing a concert at the archaeological site of Chichen Itza, a UNESCO World Heritage Site and one of the New Seven Wonders of the World. One year later, in 2010, Brightman continued touring Asia with five performances in Tokyo alone, followed by presentations in Kanazawa, Nagoya, Osaka in Japan, Macau in China and Seoul in South Korea.

On 3 November 2010, Brightman was invited to sing at the Tōdai-ji Buddhist temple complex located in the city of Nara, Japan. The temple is a listed UNESCO World Heritage Site as one of the "Historic Monuments of Ancient Nara". The concert was recorded and later broadcast nationwide by TBS network.

On 1 and 2 October 2011, Brightman made a special appearance during the finale of The Phantom of the Opera at the Royal Albert Hall; a fully staged production performed at the famous London venue—marking 25 years since the musical received its world premiere. Introduced as his "Angel of Music" by Andrew Lloyd Webber, Brightman performed the title song backed by five past, present, and Future Phantoms. The production front of an audience of 5,500 was broadcast live into cinemas around in Australia, Canada, Europe, Japan and the US.

In 2012, in conjunction with Virgin Galactic, the Brightman STEM Scholarship program (science, Technology, engineering and mathematics) was launched. It will help young women in the US pursue STEM education across their four-year college careers.

On 30 October 2013, Brightman announced through the media that she was honoured to be part of the Advisory Council for the Challenger Center, the non-profit science, Technology, engineering and maths (STEM) education organisation. Brightman also announced that she will be joined on the council. "Through my partnership with the Challenger Center, I hope to inspire in children the same wonder and excitement for space exploration that I feel myself", she commented. Challenger Center and its network of more than 40 Challenger Learning Centers engage students in hands-on experiences that strengthen knowledge in science, Technology, engineering and maths (STEM) and inspire students to pursue careers in these important fields.

In April 2014, Brightman participated as an orchestra Director in Parkinson's UK Symfunny at the Royal Albert Hall 4 June, with the aim of finding a cure for the disease.

In July 2015, the inaugural Andrew Lloyd Webber Foundation Sarah Brightman Music Scholarships were also awarded to their first recipients. The scholarships will support students in their postgraduate studies at the Royal Northern College of Music on the two-year Master of Music Programme.

Brightman was awarded the decoration 'Cavaliere' in the Order of Merit of the Italian Republic on 2 June 2016.