Born in Alhambra, California, in 1941, Don Underhill was 14 when he auditioned for and was chosen as a Mouseketeer for The Mickey Mouse Club (1955). He appeared on the show in its first season, singing and dancing in the show's many musical numbers and appearing in the opening "Roll Call". He also, with other Mouseketeers, performed in a variety of shows at Disneyland. In 1956 as the show entered its second season, he was let go, along with most of the original cast.
After his show-business career, he graduated high school and enlisted in the US Navy, serving two years. After his discharge he attended local colleges and graduated with a Bachelors Degree. He became an accountant. In 1980 he appeared on the Mouseketeers 25th Anniversary Reunion Show, his first--and last--television appearance since he left the show. In the early 1990s he retired to Summerhaven, Arizona, in the mountains near Tucson, and bought The Alpine Lodge, a resort in the town. In 2003 the lodge was destroyed in a fire that swept the town. Don decided against rebuilding, sold the property and moved to Tucson, where he resides.