Portman was born on 11 December 1960 in Haslemere in Surrey, England, the daughter of Sheila Margaret Penelope (née Mowat) Portman and Berkeley Charles Berkeley Portman.
She was educated at Charterhouse School and became interested in music from a young age, beginning composing at the age of 14.
After finishing school, Portman studied Music at Worcester College, Oxford. It was here that her interest in composing music for films began as she started experimenting with writing music for student films and theatre productions.
Rachel Portman's career in music began with writing music for drama in BBC and Channel 4 films such as Oranges Are Not the Only Fruit, Mike Leigh's Four Days in July and Jim Henson's Storyteller series.
Since then, Portman has written over 100 scores for film, television and theatre, including The Manchurian Candidate (Jonathan Demme), Oliver Twist (Roman Polanski), Hart's War (Gregory Hoblit), The Legend of Bagger Vance (Robert Redford), Beloved (Jonathan Demme), Benny and Joon (Jeremiah Chechik), Life Is Sweet (Mike Leigh), Never Let Me Go (Mark Romanek), Grey Gardens (Michael Sucsy), The Duchess (Saul Dibb), One Day (Lone Scherfig), The Vow (Michael Sucsy), Snow Flower and the Secret Fan (Wayne Wang), The Lake House (Alejandro Agresti), Infamous (Douglas McGrath), Mona Lisa Smile (Mike Newell), and The Human Stain (Robert Benton).
Portman is perhaps best known for her music soundtrack compositions in the movies Chocolat and The Cider House Rules.
Her other works include a children's opera, The Little Prince (which was later adapted for television) and Little House on the Prairie, a musical based upon the Laura Ingalls Wilder books Little House on the Prairie (2008). Portman was commissioned to write a piece of choral music for the BBC Proms series in August 2007.
Portman was the first female composer to win an Academy Award in the category of Best Musical or Comedy Score (for Emma in 1996). (Previously, female songwriters Barbra Streisand, in 1977, Buffy Sainte-Marie, in 1983, and Carly Simon, in 1989, each won Oscars, but in the category of Best Original Song). Portman was also nominated for Academy Awards for her scores for The Cider House Rules in 1999 and Chocolat in 2000.
On 19 May 2010, she was given the Richard Kirk Award at the BMI Film & TV Awards for her contributions to film and television music. Portman is the first woman to receive the honour.
Portman was appointed Officer of the Order of the British Empire (OBE) in the 2010 New Year Honours.
In 2015 Portman received the Primetime Emmy Award for Outstanding Music Composition for a Miniseries, Movie, or a Special for her work on Bessie.