Harold S. Prince, the musical theater producer and director who championed adventurous — even demanding — fare in a field long known for escapism and whose Broadway portfolio included “West Side Story,” “Fiddler on the Roof,” “Cabaret,” “Sweeney Todd,” “Evita” and “Phantom of the Opera,” died July 31 in Reykjavik, Iceland, where he was traveling. He was 91.

His death was confirmed by a publicist, Rick Miramontez. No other details were immediately available.

Known as Hal, Mr. Prince collected a record 21 Tony Awards during a career spanning seven decades. He was heralded as a visionary who saw theatrical potential in the most unlikely subject matter and who helped shepherd to the forefront unknown talents, many of them composers.

He gave major early breaks to some of the towering songwriting teams on Broadway: Richard Adler and Jerry Ross (“The Pajama Game,” “Damn Yankees”); Jerry Bock and Sheldon Harnick (“Fiddler on the Roof,” “Fiorello!,” “Tenderloin,” “She Loves Me”); and Fred Ebb and John Kander (“Cabaret,” “Kiss of the Spider Woman,” “Zorba”). He also boosted the career of young composer-lyricist Jason Robert Brown, whose “Parade” (1998) — about a notorious anti-Semitic lynching in Georgia in 1915 — won two Tonys.

Composer Stephen Sondheim — Mr. Prince’s longtime friend and game-changing collaborator — told the New York Times in 2000 that Mr. Prince was “one of the very few champions of new work in the commercial theater. New writers are interested in expanding the possibilities, and most musical theater producers are interested in shrinking them.”

Mr. Prince was 26 when he co-produced his first show on Broadway, “The Pajama Game” (1954). With songs including “Hey There” and “Steam Heat,” it brought an appealing song-and-dance style, as well as romance, to a plotline about a labor strike at a pajama factory. The combination helped secure the Tony for best musical and presaged the social concern that would characterize much of Mr. Prince’s work.

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The overnight Broadway wonder then hit another home run producing the baseball-meets-Faust story “Damn Yankees” (1955). Two years later, he provided essential backing for “West Side Story,” which retold Shakespeare’s “Romeo and Juliet” against the backdrop of warring New York street gangs.

Even the risk-embracing Mr. Prince admitted to having had reservations about the potential commercial appeal of “Fiddler on the Roof” (1964), based on Sholem Aleichem’s Yiddish stories about a shtetl milkman, Tevye, and his willful wife and daughters in czarist Russia.

He later quipped to Forbes magazine, “There are at least three million Jews in New York, and I thought that should be enough to keep the show running for a couple of seasons. But I never foresaw that Fiddler would run the way it did.” The musical, directed by Jerome Robbins and starring the magnetic Zero Mostel, won nine Tony Awards and ran for nearly eight years — a Broadway record at the time.

Mr. Prince presented the rise of fascism in Weimar Germany through the lens of a dingy, decadent nightclub with a sinister master of ceremonies in “Cabaret” (1966, with Mr. Prince producing and directing). He also mined rich political metaphors producing “Fiorello!” (1959), about New York City Mayor Fiorello La Guardia, and directing “Evita” (1978), about the scheming, image-conscious wife of Argentine dictator Juan Perón.

The Prince-Sondheim collaboration included the hit farce “A Funny Thing Happened on the Way to the Forum” (1962), directed by George Abbott and starring Mostel, before moving in daring new directions just a few years later. They largely created what many critics dubbed concept musicals — works in which the plot was constructed around themes or ideas. Narrative structure was often inventive. “Merrily We Roll Along” (1981), a tale of a broken friendship produced and directed by Mr. Prince, was told in reverse to bring bitter irony to the story’s youthfully optimistic finish.

The Sondheim-Prince partnership resulted in many shows that, while not always commercially successful at first, are today regarded as landmarks in modern musical theater. With Prince largely producing and directing, these works included “Company” (1970), a meditation on marriage; “Follies” (1971, co-directed with Michael Bennett), about a reunion of aging follies performers (and an expensive flop at the time); and “A Little Night Music” (1973), based on filmmaker Ingmar Bergman’s romantic roundelay “Smiles of a Summer Night.”

“Hal is going to be much more intrigued by problems he hasn’t solved before,” Sondheim told The Washington Post in 1991. “Pacific Overtures” (1976), about Commodore Perry’s mid-19th century opening of Japan to the West, was “a perfectly straightforward play,” Sondheim said, until Mr. Prince envisioned it in Kabuki style.

Sondheim’s idea for the genre-pushing, near-operatic “Sweeney Todd” (1979), about a barber and baker who engage in murder and cannibalism, was initially too small and darkly conceived for Mr. Prince’s taste. “It wasn’t until he thought of doing it in that epic way that he got interested,” Sondheim said. “The whole point is not to be bored.”

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Harold Smith Prince was born in New York City on Jan. 30, 1928, the only child of what he called a “privileged, upper-middle, lower-rich class, Jewish family.” His father was a Wall Street stockbroker, and his mother was an avid theatergoer. Mr. Prince was captivated by the stage when he saw Orson Welles’s 1937 Broadway staging of “Julius Caesar.”

Mr. Prince entered the University of Pennsylvania with the intention of becoming a playwright. After graduating in 1948, he hustled his way into a low-level job with Abbott, the writer-producer-director whose Broadway credits spanned the 20th century and who remained a trusted adviser until his death at 107 in 1995.

Mr. Prince’s rapid advance to Abbott’s assistant stage manager was interrupted by two years of Army service in West Germany, but he returned eager to make his own mark in theater. When the New York Times gave a rave review to Richard Bissell’s novel “7 1 /2 Cents,” he optioned it the next day.

With two partners, Mr. Prince produced a musical based on the book. They recruited Abbott to write the show with Bissell and co-direct it with Robbins, and they gave a young Bob Fosse his first job as choreographer. They also hired themselves as stage managers. “We needed the money,” Mr. Prince joked.

The producers lined up 164 investors because “none of the smart money would support us,” Mr. Prince recalled. “The Pajama Game,” starring John Raitt and Janis Paige as the romantic leads, ran for more than two years. Mr. Prince also shared his first Tony Award for best musical.

The next several years resulted in smashes (“Damn Yankees,” in 1955, harvested seven Tonys) and flops (“New Girl in Town,” 1957, a musical based on Eugene O’Neill’s waterfront melodrama “Anna Christie”). Through it all, Mr. Prince was a dervish of creative impatience.

From Abbott, he adopted the trick of moving quickly from one project to the next, setting up meetings to discuss a new show the day after an opening. “No matter what happens,” he explained to Time magazine, “you feel you are still working.”

“West Side Story” — about rival white and Puerto Rican gangs — had been germinating for years among playwright Arthur Laurents, director-choreographer Robbins, composer Leonard Bernstein and lyricist Sondheim.

In 1957, weeks before rehearsals began, a key producer withdrew amid financial concerns. Other backers, doubting the appeal of a show in which most of the lead characters die, failed to materialize. Robbins, already a marquee name, wanted out.

Sondheim called on his friend Mr. Prince, who immediately agreed to step in . He persuaded Robbins to stay and complete the show’s signature gangland ballets.

Despite solid reviews, the show was topped for best musical at the Tony Awards by “The Music Man.” Only after the 1961 film version co-directed by Robbins and Robert Wise, which dominated the Oscars, did “West Side Story” become one of the all-time champs of musical theater.

Mr. Prince began directing shows in the early 1960s — the Bock-Harnick confection “She Loves Me” was an early producing-directing credit — and had mixed luck. But he spoke of “Cabaret” as a turning point in his confidence — a show where he knew everything he wanted from the performers. He also created the musical’s atmosphere, its visceral sense of cheap debauchery.

“That’s the show that freed me,” he later told Playbill. “I remembered when I was in the Army in 1951, I went to a sleazy nightclub in Stuttgart in the basement of a bombed-out church, and there was this emcee all made up. We put that character into the show, making him represent Germany, and made him a metaphor for National Socialism.”

Enduring the stage flops

As a producer, Mr. Prince endured a long succession of high-profile flops in the late 1970s and 1980s.

Songwriters Andrew Lloyd Webber and Charles Hart helped reverse Mr. Prince’s fortunes when they asked him to direct “The Phantom of the Opera,” which opened in London in 1986 and on Broadway in 1988. He had earlier directed the Lloyd Webber-Tim Rice hit “Evita,” a rock-filled hit featuring Patti LuPone in the Broadway production, after the 1978 London debut.

In “Phantom,” Mr. Prince was credited with emphasizing the title character’s sensuality and intelligence instead of slushy melodrama, as well as giving the show what he called a “mysterious, perfumy atmosphere.”

“I wanted the show to have some depth,” he told New York magazine. “I wasn’t looking to do Dracula with music.”

His romantically dark, cinematic staging featured the spectacle of a chandelier crashing to the stage and a chase to a lake beneath the opera house. The show, still running on both sides of the Atlantic, has since become the biggest box-office draw of all time.

As a director, Mr. Prince also took a rare stab at a major revival in 1994 of Jerome Kern and Oscar Hammerstein II’s 1927 musical “Show Boat.” Mr. Prince’s version, with a cast of more than 70 and a budget of $8.5 million, ran for three years and garnered a Tony for best musical revival.

In 1963, Mr. Prince married Judy Chaplin, daughter of Oscar-winning songwriter, producer and musical director Saul Chaplin. Besides his wife, survivors include two children, Charles and Daisy, and three grandchildren.

Mr. Prince was a recipient of the Kennedy Center Honors in 1994, the National Medal of Arts in 2000 and a lifetime achievement Tony in 2006. In 2017, a career retrospective called “Prince of Broadway” had a short run on Broadway to poor reviews.

In the new century, Mr. Prince’s lifelong interest in new material was clearly out of sync, as Broadway grew crowded with movie adaptations and pop catalogue shows.

“I’m a big believer — and sometimes I feel I’m almost alone in this — that you can and should do what you want to do and bring the audience with you rather than have them lead you,” he told the Hartford Courant in 2005. “I think that’s one of the problems with the commercial theater right now. Everyone’s dealing with surveys and demographics and what worked last year and all the rest of it — and it’s a very bad way to create a show. It’s art, for God’s sake. Yes, it’s commerce, too, but you can’t lose sight of the art. Who else [but an artist] would do ‘Fiddler?’ Was the market there desperate to hear the Tevye stories?”