He had cheerfully forsaken a promising solo career for a life in chamber music and — because that life entails as much — diplomacy. (A string quartet is nothing short of a quadrilateral marriage in which the spouses rarely see eye to eye on interpretive matters.)

It was a decidedly unexpected calling for a boy who had wanted only to be a forest ranger.

Robert Nathaniel Mann was born in Portland, Ore., on July 19, 1920, into what he later described as a “very poor” family. Both of his parents were immigrants: His father, Charles, a tailor, had come from England; his mother, Anna Schnitzer, from Poland.

“My father knew nothing about music, but he used sense in going to the concertmaster of the Portland Symphony for advice,” Mr. Mann told The Toronto Star in 1995. “It was the greatest break of my life. He told my father, ‘Your son is no wunderkind, but if he practices hard, he can make a living.’ ”

When he was 9, Robert began lessons, at $1.50 each, with a teacher he recalled as an alcoholic. Two years later, the teacher was shot and killed — an actuarially unorthodox end for a classical musician.

Robert was 13 before another teacher was found, but the new teacher proved transformative.

“Up to then, I was going to be a forest ranger, hopefully in a national park,” Mr. Mann, recalling a boyhood lesson, said in “Speak the Music: Robert Mann and the Mysteries of Chamber Music,” a 2013 documentary. “But that day, I thought, ‘You know, music is very interesting.’ ”

At 18, the young Mr. Mann took up a scholarship at the Institute of Musical Art, a forerunner of the Juilliard School, in New York. The next year, he transferred to the Juilliard Graduate School (likewise a forebear of today’s Juilliard School). There, he studied composition with Stefan Wolpe and violin with Édouard Dethier, a passionate lover of chamber music.

In 1941, Mr. Mann won the violin competition of the Naumburg Foundation, which carried as its prize a debut recital at Town Hall in New York. As things fell out, the recital could scarcely have been booked for a more inopportune date: Dec. 9, 1941, two days after the Japanese attack on Pearl Harbor.