Moss was born in New York City; his father was a stage and screen actor, Arnold Moss, and his mother, Stella Reynolds gave up acting to become a soap opera writer. He attended the Browning School, a prestigious New York private school, and was #1 in his class.

He attended Princeton University, and was a member of the Princeton Triangle Club theater company. After graduating in 1963, he took a job as a production assistant at the children's television show Captain Kangaroo (he also got an offer to work for CBS News, which he later said he had turned down because "I've seen the news").

In 1969, he became the first head writer, along with a composer and lyricist, for Sesame Street. He would eventually win fourteen Emmy Awards for the show. Songs he wrote for the show include "I Love Trash", "Who Are the People in Your Neighborhood", and "Rubber Duckie". "Rubber Duckie" became a surprise mainstream hit, reaching #16 on the Billboard Hot 100 in September 1970. Moss is also credited with, among other things, creating the character of Cookie Monster, based on a puppet Jim Henson had created called "Boggle Eyes".

Moss wrote the song "Nasty Dan", which Johnny Cash sang when he appeared on Sesame Street; it later appeared on the 1975 The Johnny Cash Children's Album. In 1976, the song became a #1 hit in France for Claude François, who recorded it with French lyrics under the title "Sale Bonhomme". In 1984, Moss wrote the music and lyrics for The Muppets Take Manhattan.

In the late 1970s, Moss wrote Double Feature, a musical which received good reviews when it opened in New Haven, Connecticut. Moss worked with Mike Nichols and Tommy Tune, but when Moss became adamant about not implementing changes Nichols wanted, Nichols and Tune walked out, and the show opened off-Broadway to poor reviews in October 1981, and quickly closed. In 1993, he penned the music for a Rita and Runt song, "I'm Nobody's Mama", for Animaniacs, along with show writer Deanna Oliver.

Moss wrote many children's books, including the 1989 book "The Butterfly Jar" and the 1992 "Bob and Jack: A Boy and His Yak". He also wrote some under the Sesame Street brand name, such as "The Sesame Street Book of Poetry" and The Sesame Street Songbook.

His other works include "The Other Side of the Door" (1991), "Hieronymus White: A Bird Who Believed That He Always Was Right" (1994), "Bone Poems" (1998) and "The Dad of the Dad of the Dad of Your Dad."

Moss was credited with winning fourteen Emmy Awards, and in 1984, was nominated for an Academy Award for the music and lyrics he wrote for The Muppets Take Manhattan.^[1]

In 2007, Princeton University ranked Moss as one of its 26 most influential alumni, citing the effect of his songs and characters on the Sesame Street audience.^[1][2]

Episode 3786 of Sesame Street was dedicated in his memory.

In 1994, Moss was diagnosed with colon cancer, from which he died at his home in Manhattan, New York, on September 24, 1998, at the age of 56. He was survived by his wife, Anne Boylan; his son, Alexander Moss; and his stepson, Jonathan Boylan Smith.^[1] He died on Jim Henson's birthday.

• Joe Raposo