Robert W. Gutman, whose influential biographies of Wagner and Mozart helped upend popularly held ideas about both composers’ lives, died on Friday in the Bronx. He was 90.

His death was confirmed by Steven Walsh, his husband and only immediate survivor.

Trained in music and art history, Mr. Gutman was the author of “Richard Wagner: The Man, His Mind, and His Music” (1968) and “Mozart: A Cultural Biography” (1999).

His Wagner book, which placed its subject in the larger intellectual context of his times, infuriated idolaters, for whom the master could do no wrong.

While commending the beauty and majesty of Wagner’s compositions, Mr. Gutman also took pains to analyze his nonmusical activities — notably his profuse, turgid and virulently anti-Semitic writings, which would become a lodestar of Nazi ideology. ...