Phillips was born at the Subic Bay Naval Station in the Philippines, the son of Lucita Umayam Arañas and Gerald Amon Upchurch, a crew chief on a C-130 in the United States Marine Corps. His father was an American of Scottish-Irish and one-quarter Cherokee descent, and his mother, a native of Candelaria, Zambales, is Filipina, with distant Chinese, Japanese, Hawaiian, and Spanish ancestry.
Phillips was named after Marine legend Leland "Lou" Diamond. After his father died, he took his stepfather's surname "Phillips" as his own.
He was raised in Texas, where he attended Flour Bluff High School in Corpus Christi, graduating in 1980. He graduated from the University of Texas at Arlington with a BFA in Drama.
The first low-budget film in which he starred was called Trespasses. Phillips' big break came with the starring role in 1987's La Bamba, in which he played early rocker Ritchie Valens. Prior to his cinematic breakthrough, he starred in the Miami Vice episode "Red Tape" (March 13, 1987), portraying detective Bobby Diaz.
In 1988, Phillips co-starred with Edward James Olmos in the inner-city high school drama Stand and Deliver, in a role for which he was nominated for the Golden Globe Award for Best Supporting Actor – Motion Picture and won the Independent Spirit Award for Best Supporting Male. He plays Angel David Guzman, a cholo gangster who is inspired by his math teacher, Jaime Escalante, to excel at calculus. Working to master the subject, he develops a friendship with his teacher. Stand and Deliver was filmed before La Bamba, but it was released a year later. In 1988 and 1990, Phillips co-starred with Emilio Estevez and Kiefer Sutherland in the Western films Young Guns and Young Guns II, in which he plays Jose Chavez y Chavez, a historical Old West outlaw.
In the mid-1990s, Phillips was a vocalist with the Los Angeles-based rock group The Pipefitters.
In 1996, Phillips made his Broadway debut as the King in Richard Rodgers and Oscar Hammerstein II's The King and I. Phillips won a Theatre World Award and was nominated for both a Tony Award and a Drama Desk Award for his performance.
In 1998, he starred as Cisco, the counterpart of the main character Melvin Smiley (played by Mark Wahlberg), in the comedy-action movie The Big Hit.
He later featured a minor role in the TV sitcom George Lopez (2002–2004) as George Lopez's half-brother.
Phillips starred in four episodes of the radio series The Twilight Zone: Vol. 1, "A Kind of a Stopwatch"; Vol. 3, "The Parallel; Vol. 10, "Miniature"; and Vol. 12, "Long Live Walter Jameson".
On September 11, 2007, Phillips joined the touring troupe for Lerner and Loewe's Camelot in the role of King Arthur.
He also played a role in the first season of the TV series 24 as secret government agent Mark DeSalvo, opposite former Young Guns star Kiefer Sutherland. Phillips played the recurring role of FBI agent Ian Edgerton in the television series Numb3rs. Edgerton is an FBI tracker and sniper who works as an instructor at Quantico FBI Academy when he is not working a case in the field. Phillips won the second season of the NBC reality series, I'm a Celebrity...Get Me Out of Here!, over pro wrestler Torrie Wilson. Phillips currently hosts the weekly series An Officer and a Movie on The Military Channel. This series features various theatrical World War II dramas, with discussion breaks during the film in which Phillips interviews members of the US military and intelligence communities about details of the events that inspired each film.
Phillips had a recurring role as Colonel Telford in the Stargate Universe television series during its two-season run on the SyFy channel 2009-2011. He played the would-be commander of the Destiny expedition, who is left behind when an accident launches an unsuspecting crew into deep space. The commander works from Earth to bring the crew home, often coming into conflict with the shipborne command characters.
Phillips has been a regular poker player since college. In May 2009, Phillips placed 31st of 403 entrants in the 2009 California State Poker Championship Limit Texas hold 'em. He placed in the money at the $10,000 July 2009 World Series of Poker World Championship No Limit main event. He was eliminated as the original field of 6,494 was trimmed from 407 to 185. He entered the day in 114th place among the 407 and was eliminated on the final hand of the day finishing in 186th place and earning $36,626.
In January 2012, he was one of eight celebrities participating in the Food Network reality series, Rachael vs. Guy: Celebrity Cook-Off. On January 29, 2012, he was announced as the winner with a Zagat score of 28 out of 30, winning $50,000 for his charity.
In June 2012, Philips began co-starring in Longmire, about a modern-day sheriff played by Robert Taylor. Phillips plays Henry Standing Bear, a Native American, who is Longmire's best friend—often helping him with cases and in dealing with the reservation police who do not respect or like outsiders, especially other law enforcement.
In December 2012, he was featured in Imagine Dragons' music video for "Radioactive".
In February 2013, Phillips appeared as star of the comedy short film "Lucy in the Sky with Diamond", playing a hyperbolized version of himself known as the elusive and mysterious 'LDP' -- a renegade, spirit guide and life coach who attempts to help John (John Patrick Jordan) get over a particularly disconcerting ex-girlfriend. The award-winning short was written and directed by Joey Boukadakis.
In July 2014 he replaced the injured Jason Scott Lee in Opera Australia's Melbourne production of The King and I, reprising his role as the King of Siam, playing opposite Lisa McCune as Anna Leonowens.
In 2014, he also guest-starred in The Wiggles Rock and Roll PreSchool DVD and makes guest appearances on their TV program on ABC.
In 2016, Philips portrayed serial killer Richard Ramirez in The Night Stalker.
During the making of Trespasses, he met Julie Cypher, an assistant director. They married on September 17, 1987 and divorced on August 5, 1990. Cypher then announced she was a lesbian and became a partner to Melissa Etheridge.
He met Jennifer Tilly on the set of Shadow of the Wolf, to whom he was briefly engaged, although the two never married. He later married Penthouse model and television extra Kelly Phillips, with whom he has three daughters: Grace and Isabella (twins born in October 1997), and Lili, born in September 1999. They all reside with their mother. The two separated in 2004. Their divorce was finalized in July 2007.
He began dating makeup artist Yvonne Boismier in 2004. They married in August 2007. Their daughter, Indigo, was born in October 2007.
On August 11, 2006, he was arrested for domestic violence at his Los Angeles home following a dispute with his live-in girlfriend and future wife, makeup artist Yvonne Boismier. In December 2006, he pleaded no contest to a misdemeanor count of domestic battery and was sentenced to three years of probation. He also was ordered to undergo one year of domestic violence counseling and to serve 200 hours of community service.
Phillips has been an outspoken advocate of the Filipino Equity Act, legislation to extend the same benefits to World War II veterans from the Philippines as have been available to U.S. World War II veterans.
Phillips is very close to the Native American community. In 1990, he was adopted by the Sioux tribe, although he is Cherokee by blood. "Star Keeper" is his Sioux name. That year, he also organized a concert, The Winds of Life, to benefit Native American causes.1989 – Independent Spirit Award for Best Supporting Male (Stand and Deliver, 1988)
1989 – Golden Globe Award nominee for Best Supporting Actor – Motion Picture (Stand and Deliver, 1988)
1989 – Western Heritage Award Bronze Wrangler Theatrical Motion Picture (Young Guns, 1988). Shared with John Fusco (producer), Christopher Cain (producer), Charlie Sheen (actor), Emilio Estevez (actor), Kiefer Sutherland (actor)
1993 – Oxfam America award for his dedication toward ending world hunger
1994 – Houston International Film Festival Gold award for Best Theatrical Feature Film for Ultimate Revenge
1996 – Tony Award nominee for Best Actor on Broadway (The King And I)
1996 – Theater World Award: The King and I
1996 – New York Outer Critics Circle: Outstanding Broadway Debut Award of an Actor, The King & I
1997 – Blockbuster Entertainment Award for Favorite Supporting Actor – Adventure/Drama (Courage Under Fire, 1996)
1997 – Lone Star Film & Television Award for Best Supporting Actor (Courage Under Fire, 1996)
2001 – Filipinas magazine "Achievement award for Entertainment"
2003 – Cinemanila Film Festival Lifetime Achievement Award (Philippines)
2005 – Asia Pacific Islander Heritage Award for Excellence in Entertainment and Arts
2009 – I'm a Celebrity... Get Me Out of Here!: King of the Jungle
2012 – Rachael vs. Guy: Celebrity Cook-Off: Winner