In 1964, Forbes published a highly-respected revised edition of Thayer's Life of Beethoven, the core 19th-century biography of the composer. He also edited Beethoven's Symphony No. 5 for the Norton Critical Score series and contributed entries for The New Grove Dictionary of Music and Musicians.

Nicknamed "El," Forbes was born in Cambridge in 1917. He was a great-grandson of Ralph Waldo Emerson; another great-grandfather was 19th-century industrialist John Murray Forbes. His father, Edward W. Forbes, was professor of art history at Harvard and director of the university's Fogg Art Museum.

Forbes studied at Shady Hill School in Cambridge, Milton Academy, and Harvard— where one of his composition teachers was Walter Piston. He earned a masters there, before joining the faculty at Princeton University. In 1958 he returned to Harvard; he retired in 1984. Harvard awarded him a honorary doctorate in 2003.

Forbes had wide-ranging musical interests; he was also a fan of jazz and blues. He was 88.