She is one of a seemingly endless number of Broadway actors who consider Mr. Harnick a mentor and friend; she met him when he flew to London to see another “Fiddler” production (she played another daughter, Hodel), and quickly found herself confiding in him about her father’s recent death.

“He’s a patron, a grandfather and a guardian angel — he always wants to know about your life, how you feel about the art you’re making, how you feel about the business and whether you’re growing,” she said. “It sounds corny, but it’s sort of a miracle of miracles that Sheldon is still here to see the lens through which each revival gets viewed as the world changes – the Holocaust, the women’s liberation movement, whatever is happening in that moment.”

Mr. Harnick’s skill, actors and directors say, is in writing lyrics that are deceptively straightforward. Barbara Cook, the Broadway actress and singer, described Harnick lyrics as “like conversation — he’s not just trying to rhyme things.” Harold Prince, the producer, said, “They’re smart and sharp, and character delineation is very strong.” And Todd Haimes, the longtime artistic director of the Roundabout Theater Company, cited the lyrics of “She Loves Me,” which his theater is reviving this season, saying “Everything is just perfect — the rhymes don’t feel forced, it’s emotional, it’s funny, and it moves the story forward in an organic way.”

Writing lyrics is not a profession most little boys dream of, but Mr. Harnick traces his career back to his Chicago childhood, when his mother would write doggerel for birthdays, bar mitzvahs and weddings. His sister began writing serious poetry; young Sheldon opted for comic verse.

Success, of a sort, came early — his poems were published in his grammar and high-school newspapers; a friend invited him to work on song parodies and theatrical sketches; he acted in an Off Loop production of “a terrible play” called “Those Extraordinary Greeks” that was shut down after one performance.

He was drafted into the Army during World War II; during three years of service, he never saw combat (“I do have a bayonet wound, but it came from trying to open a can of peanuts with my bayonet”), but he did meet a theatrical agent, Saul Lerner, and when there was time he would wrote songs for comrades to send girlfriends.