Sondheim, whose eight lifetime Tony Awards surpassed the total of any other composer, started early, learning the art of musical theatre when he was just a teenager from "The Sound of Music" lyricist Oscar Hammerstein II.
"Hamilton" creator Lin-Manuel Miranda, who was mentored by Sondheim, has called him musical theatre's greatest lyricist.
Sondheim's most successful musicals included "Into the Woods," which opened on Broadway in 1987 and used children's fairy tales to untangle adult obsessions, the 1979 thriller "Sweeney Todd" about a murderous barber in London whose victims are served as meat pies, and 1962's "A Funny Thing Happened on the Way to the Forum," a vaudeville-style comedy set in ancient Rome.
"I love the theatre as much as music, and the whole idea of getting across to an audience and making them laugh, making them cry - just making them feel - is paramount to me," Sondheim said in a 2013 interview with National Public Radio.
Several of Sondheim's hit musicals were turned into movies, including the 2014 film "Into the Woods," starring Meryl Streep, and the 2007 "Sweeney Todd" with Johnny Depp. A new film version of "West Side Story," for which Sondheim wrote the lyrics for Leonard Bernstein's music, opens next month.
His songs were celebrated for their sharp wit and insight into modern life and for giving voice to complex characters, but few of them made the pop charts.
'CLOWNS' HIT
He had a hit, however, with the Grammy-winning "Send in the Clowns" from his 1973 musical "A Little Night Music." It was recorded by Frank Sinatra, Sarah Vaughan and Judy Collins, among others.
One of Sondheim's greatest triumphs was his Pulitzer Prize for the 1984 musical "Sunday in the Park with George," about 19th-century French Neo-Impressionist artist Georges Seurat.
As Sondheim collected accolades, New York City's Broadway theatre industry underwent many changes. It had a key role in American culture through the 1950s, with many Broadway songs making the pop charts, but lost significance as rock music gained a hold on the public starting in the 1960s.
Increasingly, musicals borrowed material from television and movies, instead of the other way around, composer Mark N. Grant wrote in his book "The Rise and Fall of the Broadway Musical."
Sondheim shared the view that Broadway had experienced decline, expressing it repeatedly in interviews.
"There are so many forms of entertainment, theatre is becoming more marginalised," he told British newspaper The Times in 2012.
But Broadway musicals also became more artistic and Sondheim played a key role in their evolution, critics said. He explored such weighty topics as political assassinations in "Assassins," the human need for family and the pull of dysfunctional relationships in "Into the Woods," social inequality in "Sweeney Todd," and Western imperialism in "Pacific Overtures."
He also developed new methods for presenting a play. Instead of telling a story from beginning to end, he would jump backward and forward in time to explore a single theme. It was called the "concept musical."
Broadway audiences were introduced to Sondheim in 1957 when he wrote the lyrics for "West Side Story" to go with Leonard Bernstein's music and it became an American classic. The story about a love affair between a Puerto Rican girl, Maria, and a white boy, Tony, in working-class Manhattan was turned into an Oscar-winning film in 1961. The central characters expressed their infatuation in the songs "Maria," "Somewhere" and "Tonight."
