Michael Tilson Thomas, whose trailblazing leadership of the San Francisco Symphony spurred American classical music’s westward expansion, announced on Tuesday that he would step down as its music director in 2020, after his 25th season.

Under his baton, the ensemble secured its place among, and helped redefine the role and repertory of, America’s top orchestras. Its success, and that of the Los Angeles Philharmonic to the south, exploded the myth of an industry dominated by venerable East Coast ensembles.

Mr. Thomas, 72, has been lauded for conducting both Mahler and his beloved “mavericks” — offbeat American composers including Charles Ives, John Cage, Morton Feldman, Henry Cowell and Lou Harrison. His lavishly produced Keeping Score series was perhaps the best music education on television since his mentor, Leonard Bernstein. And under his leadership, San Francisco opened a small nightclub-like space for unconventional programming called SoundBox that is the envy of other orchestras.

Widely known as M.T.T., Mr. Thomas will stay on as the artistic director of the New World Sympony, the training academy in Miami he co-founded — and he will not disappear from Davies Symphony Hall in San Francisco. He said in a telephone interview that as music director laureate, he would continue to lead the orchestra for at least four weeks a season. Here are edited excerpts from the conversation.