Born in New York City, Schonberg grew up there, graduated from Brooklyn College in 1937, and did graduate studies at New York University. In 1939 he became a record critic for American Music Lover magazine (later renamed the American Record Guide). The son of David and Mini Schonberg, he had a brother (Stanley) and sister (Edith).

Schonberg joined The New York Times in 1950. He rose to the post of senior music critic for the Times a decade later. In this capacity he published daily reviews and longer features on operas and classical music on Sundays. He also worked effectively behind the scenes to increase music coverage in the Times and develop its first-rate music staff. Upon his retirement as senior music critic in 1980 he became cultural correspondent for the Times.

Schonberg was an extremely influential music writer. Aside from his contributions to music journalism, he published 13 books, most of them on music, including The Great Pianists: From Mozart to the Present (1963) — pianists were a specialty of Schonberg — and The Lives of the Great Composers (1970; revised 1981, 1997) which traced the lives of major composers from Monteverdi through to modern times.

Criticisms of Bernstein

Schonberg was highly critical of Leonard Bernstein during the composer-conductor's eleven-year tenure (1958–1969) as principal conductor of the New York Philharmonic. (These were the years that Bernstein's name became a household word because of his appearances on CBS in the Young People's Concerts.) He obliquely accused Bernstein of showing off by using exaggerated gestures on the podium, and more directly, of conducting a piece in a way that made its structure overly obvious to audiences, and, by implication, to music critics, who supposedly already knew all about the particular piece being conducted.

One of his most famous criticisms of Bernstein was written after the famous April 6, 1962 performance before which Bernstein announced that he disagreed with pianist Glenn Gould's interpretation of Brahms' Piano Concerto No. 1, but was going to conduct it anyway because he found it fascinating. Schonberg chided Bernstein in print, suggesting that he should have either backed out of the concert or imposed his own will on Gould, and called him "the Peter Pan of music." In the chapter on Bernstein in his 1967 book The Great Conductors, Schonberg quotes the remark, but does not admit that he was the critic who had made it. It was commonly said that Schonberg was carrying an ongoing feud with Bernstein.

After Bernstein's regular tenure at the New York Philharmonic ended, however, Schonberg seemed to mellow in his attitude toward him and actually began to praise his conducting, stating in his book The Glorious Ones that "with age, came less of a need to prove something," and that "there were moments of glory in his conceptions."

A devoted and skilled chess player, he covered the championship match between Boris Spassky and Bobby Fischer in Reykjavík, Iceland in 1972. One of Schonberg's books not on music was Grandmasters of Chess. He also reviewed mysteries and thrillers for The New York Times under the pseudonym Newgate Callendar.

Schonberg was also an avid golfer, though a poor one by his own estimation. He co-authored the book How To Play Double Bogey Golf (1975) along with Hollis Alpert, founder of the National Society of Film Critics, and fellow author Ira Mothner. Schonberg, Mothner and Alpert played golf frequently together, according to the book.

In 1987, it was announced that Schonberg was assisting Vladimir Horowitz in the preparation of the pianist's memoirs. Although the project was never completed, Schonberg's biography of Horowitz was published in 1992.

Schonberg died in New York City on July 26, 2003, at the age of 87. In his obituary notice in The New York Times the next day, Allan Kozinn wrote that "as a music critic Harold Schonberg set the standard for critical evaluation and journalistic thoroughness."