Gunther Schuller, composer, conductor, writer and educator, in 1967. (AP)

Gunther Schuller, a Pulitzer Prize-winning composer whose breadth of musical expertise ranged from Bach to Duke Ellington and who sought to create what he called a “Third Stream” of music by blending classical music and jazz, died June 21 at a hospital in Boston. He was 89.

The death was confirmed by a son, Ed Schuller. The cause was complications from leukemia.

Mr. Schuller began his career playing the French horn in orchestras, but he constantly explored other forms of musical expression throughout his life.

As a composer, he wrote more than 200 pieces of music. As a conductor, he led ensembles as varied as the Berlin Philharmonic, a Grammy-winning ragtime band and the Smithsonian Jazz Masterworks Orchestra in Washington. And as a jazz educator in the 1960s, he made the New England Conservatory the first classical music school to grant degrees in jazz studies. He also published several books, including two acclaimed histories of jazz.

“Gunther Schuller may well be the ultimate Renaissance man of 20th-century music,” critic Leonard Feather wrote in the Los Angeles Times in 1989. “His brilliant mind knows no barriers.”

Mr. Schuller’s grandfather was a conductor in Germany, and his father played in the violin section of the New York Philharmonic for more than 40 years.

By the time he was 15, the young Mr. Schuller was already a substitute French horn player for his father’s orchestra, but he was also beginning a lifelong infatuation with jazz.

“I couldn’t figure why my parents were telling me that jazz was degenerate, unimportant music,” he told the Chicago Tribune in 1989. “Because when I started hearing it, especially Duke Ellington, I knew right away that it was great music — extraordinary harmonies, extraordinary orchestrations, extraordinary rhythms and invention and imagination.”

While playing with the Cincinnati Symphony in the 1940s, Mr. Schuller dashed out to jazz clubs after his concerts. Later, living in his native New York, he led a busy and divided musical life.

For 14 years, he was a member of the Metropolitan Opera orchestra, but he also moonlighted in Broadway show bands and jazz groups. He performed on the groundbreaking “Birth of the Cool” recordings with Miles Davis and Gerry Mulligan in the late 1940s.

He wrote music that might best be described as Ellington-meets-Arnold Schoenberg, in which classical rigor was enlivened by the vitality of jazz. At first, some observers were puzzled by what Mr. Schuller called his “Third Stream” music.

“Unfortunately, the stream never seemed to be flowing anywhere,” a Time magazine critic wrote in 1961.

Mr. Schuller wrote for unusual instrumental combinations, such as four double basses or four cellos. He composed concertos that featured the French horn, the contrabassoon or various percussion instruments. Over the years, such works as “Seven Studies of Paul Klee” (1959), “Symphony” (1965) and “The Visitation,” a 1966 opera, began to gain a foothold, and he conducted his compositions, as well as standard classical repertoire, with orchestras all around the world.

In 1964, Mr. Schuller began an academic career, first at Yale University, then at the New England Conservatory, which he led as president from 1967 to 1977. In 1973, he conducted a conservatory ensemble that won a Grammy Award for an album of ragtime music by Scott Joplin.

When not composing, conducting or leading a conservatory, Mr. Schuller was writing.

His 1962 book about the French horn, “Horn Technique,” remains a standard text, and he published a 500-page book about conducting in 1997.

He also wrote two books about the music that captivated him at a young age, “Early Jazz” (1968) and “The Swing Era” (1989), which were praised for the literary flair and intricate musical analysis.

In “Early Jazz,” Mr. Schuller explained why “the spectacular cascading phrases” of Louis Armstrong’s trumpet solo on “West End Blues” in 1928 produced such a monumental breakthrough in music:

“He established the general stylistic direction of jazz for several decades to come. Beyond that, this performance also made quite clear that jazz could never again revert to being solely an entertainment or folk music. The clarion call of ‘West End Blues’ served notice that jazz had the potential capacity to compete with the highest order of previously known musical expression.”

Gunther Alexander Schuller was born Nov. 22, 1925, in Queens. He studied at a private school in Germany in the 1930s, then attended high school and specialized music programs in New York without receiving a degree, not even a high school diploma.

Mr. Schuller held positions with several summer music festivals, most notably at the Tanglewood Music Center in Massachusetts from the 1960s to the 1980s.

Never forgetting his early classical training, Mr. Schuller began a long association with the Spokane Symphony Orchestra in Washington state in the 1980s. He was artistic director of the Northwest Bach Festival from 1993 to 2013.

Mr. Schuller’s wife of 44 years, singer and pianist Marjorie Black, died in 1992. He dedicated a composition to her memory, “Of Reminiscences and Reflections,” which received the Pulitzer Prize in 1994.

Survivors include two sons, jazz musicians Ed Schuller and George Schuller, both of Brooklyn.

During his career, Mr. Schuller performed with such a varied musical cast as Arturo Toscanini, Maria Callas, Dizzy Gillespie, Frank Sinatra, Igor Stravinsky and Ethel Merman.

He received a MacArthur Foundation “genius grant” in 1991 and was named a National Endowment for the Arts Jazz Master in 2008. He published an autobiography, “A Life in Pursuit of Music and Beauty,” in 2011.

Throughout his life, Mr. Schuller said he got by on four to six hours of sleep a night.

“If you slice away the two hours at night and two hours in the morning that other people are sleeping,” he said in 2011, “you can get a lot done over a period of 70 years.”