Bratt was born in San Francisco, California, the son of Eldy (née Banda), a nurse, and Peter Bratt Sr., a sheet metal worker. His mother is an Indigenous activist of the Quechua ethnic group; born in Peru, she moved to the United States at age 14. His father was American and had Austrian, English, and German ancestry. They married December 30, 1960 in San Francisco, but divorced in September 1967. Bratt's paternal grandfather, George Cleveland Bratt (March 5, 1893 – March 29, 1984), was a Broadway actor. He married Bratt's paternal grandmother, Wiltrude Hildner, on August 6, 1920 in Detroit, Michigan.
Bratt attended Lowell High School in San Francisco, where he was a member of the Lowell Forensic Society. Bratt earned a B.F.A. at the University of California, Santa Barbara in 1986, where he also joined the Lambda Chi Alpha fraternity. Although accepted into the M.F.A. program at the American Conservatory Theater in San Francisco, he left before receiving his degree to star in the television film Juarez (1987). As a child, Bratt went with his mother and siblings to participate in the 1969 Native American occupation of Alcatraz.
One of Bratt's first television series was Nasty Boys, based on a film by the same name in which he appeared, which aired in 1989 on NBC. His best-known role has been that of Detective Reynaldo Curtis on the television show Law & Order. In 1999, he was nominated for the Primetime Emmy Award for Outstanding Supporting Actor in a Drama Series for his work on the series.
His more popular films include Miss Congeniality, Blood In Blood Out and Traffic. Bratt often portrays Hispanic characters.
On June 23, 2009, Bratt appeared on The View to promote The Cleaner. On October 23, 2009, it was announced that Bratt would return as Detective Curtis on Law & Order. Curtis reunited with his former boss, Lt. Anita van Buren (S. Epatha Merkerson), in the episode that aired on December 11, 2009. He left the show that same year to continue his film career.
In 2009, Bratt performed in The People Speak, a documentary feature film that uses dramatic and musical performances of the letters, diaries, and speeches of everyday Americans, based on historian Howard Zinn's A People's History of the United States. In 2012, Bratt was passionate about his opportunity to play a Tlicho Indian in the film The Lesser Blessed, a project dear to his heart because of his own Native background. He voiced El Macho, the main antagonist, in Despicable Me 2, and reprised his role from Cloudy with a Chance of Meatballs as Manny the cameraman in Cloudy with a Chance of Meatballs 2.
Today, due to his activism during the American Indian Movement and pride in his Indigenous American heritage, Bratt is an active supporter of such Native American causes as the American Indian College Fund and We Shall Remain, a mini-series and multi-media project, narrated by Bratt, that establishes Native history as an essential part of American history from PBS' acclaimed series American Experience. Bratt has for years been a strong supporter and board member of San Francisco Bay Area's Friendship House Association of American Indians and Native American Health Center.
In 1998, Bratt began dating actress Julia Roberts. He escorted her to the 2001 Academy Awards ceremony, at which she won the Academy Award for Best Actress. Four months later, they announced that they were no longer a couple.
In 2002, he (along with Priscilla López) received the Rita Moreno HOLA Award for Excellence from the Hispanic Organization of Latin Actors (HOLA).
In 2002, he began dating and then married his pregnant girlfriend, actress Talisa Soto, on April 13, 2002 in San Francisco. The two met ten years earlier during the casting audition of Blood In Blood Out (1993) and afterward they saw each other on and off. It was not until the filming of Piñero (2001) that they began to develop a relationship. Their first child, daughter Sophia Rosalinda Bratt, was born on December 6, 2002; their second child, son Mateo Bravery Bratt, was born on October 3, 2005.
His brother, Peter Bratt, wrote and directed the 1996 film Follow Me Home, casting Benjamin as Abel. In 2009, Peter wrote and directed the independent film La Mission, starring Benjamin as Che Rivera, an inhabitant of the Mission District.