As a vocalist, Charles, along with Julia Rinker Miller, is known for singing the theme song to the television series Three's Company ("Come and Knock on Our Door"). As a songwriter, Charles is best known for the choral anthem "Fifty Nifty United States," in which he set the names of the states to music in alphabetical order. It was originally written for The Perry Como Show.He is also known for "Letters, We Get Letters," also originally written for The Perry Como Show and later used on Late Show with David Letterman.
At the age of 89 years, he continued to serve as a musical consultant to television programs, most notably for the last 24 years on the Kennedy Center Honors. Charles is acknowledged as an authority on American popular music.