Elliott Carter, the Pulitzer Prize-winning American composer who fused European and American modernist traditions in seminal but formidable works, and who lived to hear ovations for music that was once thought to be anything but listener-friendly, died Nov. 5 at his home in New York City. He was 103.

His assistant, Virgil Blackwell, confirmed the death but did not disclose an immediate cause.

Mr. Carter’s career was like some of the towering cathedrals of Europe: so long in the making that it reflected the dramatic shifts in artistic style that take place over a century. A late bloomer — he didn’t find his mature voice, or the style for which he was best known, until age 40 — Mr. Carter eventually received acclaim by some critics and composers. Igor Stravinsky was credited with calling Mr. Carter’s “Double Concerto for Harpsichord, Piano and Two Chamber Orchestras” (1961) the first American masterpiece.

Much of Mr. Carter’s music was difficult to play, difficult to listen to and, judging by the slow pace of Mr. Carter’s output, difficult to write. Yet it also embodied a certain simplicity. As Mr. Carter aged, he emphasized the connections between his music and the world around it. He said that he sought to represent the pace of the 20th century: the acceleration and deceleration of an airplane rather than the regular beats, and horses’ hooves, of 18th- and 19th-century music.

Mr. Carter experimented most notably with meter, or rhythm, and challenged audiences to follow multiple instruments that played simultaneously to different beats.

“A piano accelerates to a flickering tremolo as a harpsichord slows to silence,” wrote composer and musicologist David Schiff, describing Mr. Carter’s music. “Second violin and viola, half of a quartet, sound cold, mechanical pulses, while first violin and cello, the remaining duo, play with intense expressive passion. Two, three or four orchestras superimpose clashing, unrelated sounds. A bass lyrically declaims classical Greek against a mezzo-soprano’s American patter.”

Mr. Carter said that his music presented society as he hoped it would be: “A lot of individuals dealing with each other, sensitive to each other, cooperating and yet not losing their own individuality.”

Mr. Carter continued composing until shortly before his death, his works ranging from ballets to vocal, instrumental, chamber and orchestral pieces. At age 90, he premiered his first opera, appropriately called “What Next?.” The program for his 100th birthday celebration at New York’s Carnegie Hall included a new work, “Interventions,” conducted by James Levine with Daniel Barenboim as soloist. It was an impressive showing for a composer described earlier in his career as “a musical loner.”

Elliott Cook Carter Jr. was born Dec. 11, 1908, to a prosperous family in New York City. He was able to identify all the music in his parents’ collection before he learned to read.

Mr. Carter attended the private Horace Mann School in New York but spent much of his childhood in Europe; his father, a pacifist lace importer, first took him there to show him the destruction wrought by World War I. The family’s travels helped expose Mr. Carter to the music of revolutionary composers such as Stravinsky, Alexander Scriabin and Arnold Schoenberg — three men who helped determine that Mr. Carter would not grow up to be a lace importer, as his family had hoped. Mr. Carter often said that Stravinsky’s “Rite of Spring,” which he heard as a teenager at Carnegie Hall, inspired him to become a composer.

As an undergraduate at Harvard University, Mr. Carter studied literature. It remained an important part of Mr. Carter’s life: In the 1970s, he wrote a cycle of vocal music based on the writings of poets including Robert Lowell, Elizabeth Bishop and Hart Crane.

In the 1930s, after earning a master’s degree in music at Harvard, Mr. Carter took a step that was virtually de rigueur for a generation of American composers: He went to Paris to study with Nadia Boulanger.

“She wasn’t encouraging if you wrote very dissonant music,” Mr. Carter told the Guardian in 2006. “But, meanwhile, the world of music had changed. It wasn’t hard to think when we saw pictures of Hitler that it was expression that had gone on and produced such a terrible result in Germany, that it was a working out of that kind of extravagance that had become terrifying. So we thought that it was time to be more orderly and more consciously beautiful, and neoclassicism did seem to have a perfect logic about it.”

Returning to the United States in the late 1930s, Mr. Carter initially worked in the traditional mold of other Boulanger students, creating neoclassical, approachable, “American” works such as the ballet “Pocahontas,” which had its premiere in 1939. That same year, he married sculptor Helen Frost-Jones. She died in 2003. Survivors include a son, David Carter of Spencer, Ind.; and a grandson.

In the mid-1940s, after his “Holiday Overture” was rejected by the Boston Symphony, Mr. Carter moved away from so-called approachability, writing the “Piano Sonata” in 1945-6, the “Cello Sonata” in 1948 and then in 1950-1, the “String Quartet No. 1,” which was considered his first real breakthrough. The sprawling 40-minute work probed the idea of multiple perspectives in a single composition and put Mr. Carter on the map.

A performance of the quartet in Rome won the composer a good reputation in Europe — fame cemented in the 1960s and early ’70s by William Glock, the controller of music for BBC, who admired Mr. Carter’s works and played them on the radio.

If the first quartet won him acclaim in Europe, then “String Quartet No. 2” sealed his reputation in the States. Mr. Carter imagined each instrument as an individual. The first violin, he said, was intended to be “fantastic, ornate and mercurial”; the second was “laconic and orderly”; the viola was “expressive”; and the cello was “somewhat impetuous.” The work won him the first of his two Pulitzer Prizes in 1960; the second was for “String Quartet No. 3” in 1973.

Interest in Mr. Carter and his music surged as he aged, bringing a kind of mainstream attention few living composers ever receive. The subject of at least two documentaries — “A Life in Music” (1983) and “A Labyrinth of Time” (2004) — he was featured on mainstream talk shows leading up to his centennial.

Mr. Carter remained actively involved in the performance of his own work, discussing in great detail with instrumentalists the nuances of a given piece. As he got older, however, an early bedtime limited his concert-going.

In addition to his two Pulitzer Prizes, Mr. Carter’s awards include the National Medal of Arts, the Edward MacDowell Medal and two Guggenheim fellowships. He taught at Juilliard as well as at Columbia, Yale and Cornell universities, among other places.

Given Mr. Carter’s continual acclaim during the last six decades of his life, he had no reason to be anything other than sanguine about his work. But he knew that he was writing for a specialized crowd.

“As society evolves,” he once said, “people will have to become much cleverer and much sharper. And then they will like my music.”

Staff writer Emily Langer contributed to this report.