Glenn started playing professionally at age 16 in a piano bar, subsequently playing with blues and jazz musicians including Sonny Rhodes, Johnny Copeland, and Richie Cole. In 1987 he joined Blood Sweat and Tears, where he remained until 1995, leaving to join Ween. In 2006, he resumed his membership in Blood, Sweat and Tears. He is also a vocalist and keyboardist for Scott Rednor and His Band and The Happy Dog.
He has a son named Charles Glenn McClelland who is also a musician.