Foxwell was born on 18 April 1988 at the Queen Charlotte's Hospital in Hammersmith, London. The only child of her parents, Richard and Maria Foxwell, she grew up in the suburb of Isleworth, London Borough of Hounslow. Foxwell attended St Mary's and St Benedict's primary school before going on to St Mark's Catholic School. She then attended Uxbridge College where she studied sociology, psychology, English, and drama, achieving two A grades and two B grades at AS-level.
Foxwell was offered a place to read sociology at the University of Central Lancashire in Preston. She decided not to go to university, instead accepting a temporary position as a bookmaker's clerk in Harrow.
On the morning of 7 July 2005, Foxwell travelled from her home in Isleworth to her temporary employment at a William Hill bookmakers in Harrow, approximately an hour and a half commute each way. She travelled on a bus from Hounslow to Turnham Green Underground station, where she waited for about fifteen minutes before boarding a District line train to Edgware Road, arriving three minutes before 9:00 am.
Foxwell left Edgware Road and began walking towards Harrow Road but stopped to take a break and sat on a bench in Hyde Park. She spoke by telephone to her mother as she sat on the bench, and the bus that she was waiting for passed her stop.
Foxwell caught a bus and returned to Edgware Road. It was reported that she bought a packet of sandwiches from a nearby Marks & Spencer store and a newspaper from the shop at the Underground station. She then caught a No. 30 bus at approximately 9:40 am; the bus travelled south down Edgware Road and was caught in heavy traffic. At 9:47 am, the bus halted on the Edgware Road near Tavistock Square, and a bomb detonated on the upper deck of the bus, killing Foxwell. Two weeks after the bombing, it was revealed that Foxwell did not die immediately, but survived for at least 10 minutes after the bomb exploded. It emerged at the inquests in October 2007 into the July 7 bombings that she was still trying to move after the blast, before finally going into what appeared to be a seizure.
The London bombings brought the 2012 Summer Olympics security into concern.
Sarah Foxwell is believed to have been the last victim of the 7 July 2005 attacks, as it is thought she lived for the longest period of time after being injured. The Tavistock Square memorial to the 7 July 2005 attacks was unveiled in July 2009, and includes the names of all the victims, including Sarah Foxwell.