Howard was born in Los Angeles. He is from a musical family; his grandmother was the Pittsburgh Symphony's concertmaster and Violinist during the 1930s and '40s.

After Howard left college, he joined a short-lived rock band called Mama Lion. The band was led by Neil Merryweather and featured vocalist Lynn Carey, Coffi Hall on bass, and Rick Gaxiola on guitar. Mama Lion recorded two full-length albums. Members of Mama Lion also formed the band Heavy Cruiser with Merryweather singing lead, recording two albums in the Heavy Prog Psyche genre. He then worked for a couple of years as a session musician with artists including Diana Ross, Ringo Starr, and Harry Nilsson. In the early 70s, he described himself as being "dirt poor", until his big break in 1975 when his manager got him an audition with Elton John. He joined John's band and toured with them as keyboardist during the late 70s and early 80s. He was part of the band that played Central Park, New York, on September 13, 1980. Howard also arranged strings for several of John's songs during this period including the hits "Don't Go Breaking My Heart" and "Sorry Seems to Be the Hardest Word", and played additional keyboards and synthesizers on studio albums including Rock of the Westies (1975), Blue Moves (1976), 21 at 33 (1980), and The Fox (1981).

In 1982, Howard was featured on Toto IV as the strings Conductor and orchestrator for "I Won't Hold You Back", "Afraid of Love", and "Lovers in the Night". A year later, he released the live album James Newton Howard and Friends, which featured Toto's David Paich (keyboards), Steve Porcaro (keyboards), Jeff Porcaro (drums), and Joe Porcaro (percussion).

In 1983, Howard was co-producer, musician (keyboards), and orchestrator of Riccardo Cocciante's album Sincerità.

After briefly touring with Crosby, Stills and Nash, he took an opportunity brought to him by his manager to write a film score for a small-time movie. This career move would lead to his becoming a successful film music Composer. During this early foray into film music, he did not entirely abandon his previous musical path and returned for a brief collaboration with Elton John on his Tour De Force of Australia in the fall of 1986. He conducted both his own and Paul Buckmaster's arrangements during the second half of the set, which focused on orchestrated performances of selected songs from the Elton John catalog.

Howard scored the surprise blockbuster romantic comedy Pretty Woman (1990) and received his first Academy Award nomination for his score for Barbra Streisand's drama The Prince of Tides (1991). Setting the musical mood for numerous films throughout the decade, Howard's skills encompassed a plethora of genres, including four more best original score Oscar nominations, for the Harrison Ford action feature The Fugitive (1993), the Julia Roberts romantic comedy My Best Friend's Wedding (1997), M. Night Shyamalan's The Village (2004), and Michael Clayton (2007). In addition, Howard scored the Western epic Wyatt Earp (1994), Kevin Costner's Waterworld (1995), and Primal Fear (1996). His collaborations on songs for One Fine Day (1996) and Junior (1994) garnered Oscar nominations for Best Song. Along with scoring small-scaled, independent films such as Five Corners (1988), Glengarry Glen Ross (1992), and American Heart (1993), Howard proved equally skilled at composing for big-budget Hollywood spectacles, including Space Jam (1996), Dante's Peak (1997) (theme only – score was composed by John Frizzell), and Collateral (2004). He has also scored three Disney animated feature films: Dinosaur (2000), Atlantis: The Lost Empire (2001) and Treasure Planet (2002). Although he concentrates primarily on films, Howard has also contributed music for TV, earning an Emmy nomination in 1995 for his theme to NBC's ratings smash ER (Howard also scored the two-hour pilot); he also provided the themes for The Sentinel and Gideon's Crossing, winning an Emmy for the latter.

He has scored all of Shyamalan's suspense thrillers, The Sixth Sense (1999), Unbreakable (2000), Signs (2002), The Village (2004), Lady in the Water (2006), The Happening (2008), and The Last Airbender (2010), notably dropping the intense, yet subtle, opening credit music for The Sixth Sense from the corresponding Soundtrack album.

On October 14, 2005 Howard replaced Howard Shore as Composer for King Kong, due to "differing creative aspirations for the score" between Shore and Director Peter Jackson. The resultant score earned Howard his first Golden Globe nomination for Best Original Score. His work on Michael Clayton earned him an Oscar nomination. He followed in 2008 with his eighth Oscar nomination for Edward Zwick's Defiance. He also collaborated with Hans Zimmer on the scores for Batman Begins and its record-breaking sequel The Dark Knight.

In May 2008, he was made an Honorary Member of the Royal Academy of Music.

In 2009, he was awarded a Grammy alongside Hans Zimmer for the Soundtrack to The Dark Knight.

In September 2010, he was appointed visiting professor of media composition at the Royal Academy of Music in London.

From 2012-2015, James Newton Howard scored the music for the critically acclaimed Hunger Games franchise.

In 2014, Howard scored two Academy Award nominated films, Nightcrawler and Maleficent.

In October 2015, he was honored with the Max Steiner Film Music Achievement Award during the annual Hollywood in Vienna concert.

On April 7, 2016, it was announced that Howard would compose the score for the upcoming fantasy drama Fantastic Beasts and Where to Find Them, a spin-off/prequel of the Harry Potter film series. In November 2016, Howard confirmed his return as Composer for the sequel, Fantastic Beasts: The Crimes of Grindelwald.