In late summer of 1936, Barber and fellow composer Gian-Carlo Menotti rented a cabin near St. Wolfgang near Salzburg in Austria. There Barber wrote his only string quartet. It was introduced the following December in Rome by the Pro Arte String Quartet.

Later, Arturo Toscanini commissioned a new work from Barber for his newly formed NBC Symphony Orchestra. Barber sent him his First Essay for Orchestra, and also the Adagio for Strings, an arrangement of the first half of the second movement of the string quartet. Toscanini sent the scores back without comment.  Barber assumed the great conductor didn't like the works, and cast about for someone else to play them.

That summer Menotti visited Toscanini, who inquired why Barber was absent. Menotti replied that his friend was sick. “I don't believe that,” said Toscanini. “He's mad at me. Tell him not to be mad. I'm going to play one of his pieces, I'm going to play both.” The maestro had memorized the music. Toscanini and the NBC Orchestra introduced both works on November 5, 1938.

The Adagio was an instant hit. Besides performances as a concert piece, it has been used as a memorial dirge. The Adagio was played at the funerals of presidents Roosevelt and Kennedy, and after Barber's death in 1981.

“It is a knockout!” Barber said when he finished the original version of the Adagio, and commentators have been trying to explain its appeal ever since. William Schuman called it “a perfect piece of music…. I think it works because it's so precise emotionally. The emotional climate is never left in doubt. It begins, it reaches its climax, it makes its point, and it goes away.” For Virgil Thomson, “I think it's a love scene…a detailed love scene…a smooth successful love scene. Not a dramatic one, but a very satisfactory one.”