He was born Donald Lawrence Underhill in Alhambra, California, the older of two boys. Disney press releases said Don was a Boy Scout who enjoyed model boats and planes, golf, tennis and skating, and wanted to one day become an architect. According to Paul Petersen's book, Don had worked professionally before being hired for the show, though he didn't provide details. How Don came to audition for the show is unknown to me, though, since he was from Alhambra, it's tempting to speculate that he was another of Burch Mann's students.
Don was a quiet, self-effacing sort of guy, who seldom smiled and never mugged for the cameras. A capable dancer, he had very few solo performances. One of those was a short dance piece, done to the tune of The Irish Washerwoman, where Don is pitted against Bobby and Bonni. Don was often grouped in a quartet with them and Darlene for singing and skits, as they were the four oldest kids.
Like the other kids, Don had to wear his share of costumes, including a lion suit for Circus Day Roll Call and a jet fighter for Anything Can Happen Day. Because he was the second oldest boy that season, could dance, and was relatively tall, he was placed on the Red Team, and took part in Roll Call each day. However, he never seemed quite comfortable with the limelight, and later in the season was switched to the Blue Team, possibly in a swap with Tommy Cole
or Dennis Day
As the filming finished for the first season in October 1955, the Mouseketeers were shifted from the Burbank studio to Disneyland, for the Mickey Mouse Club Circus. From early November 1955 to early January 1956 (including Hanukkah, Christmas and New Year's Day), the kids performed various acts in multiple daily shows. Don took his turn at this, and like the other boys had to wear a sort of Peter Pan outfit. In mid-January 1956, most Mouseketeers, including Don, were released from their contracts.
After leaving the show in January 1956, Don returned to Alhambra High School. He lettered in Tennis and was president of the Junior Optimists, and graduated near the top of his class. In November 1959 he joined the Navy for a two-year hitch. After leaving the service in 1961, he enrolled in Woodbury College (now Woodbury University) in Los Angeles. He majored in Accounting, and graduated with a Bachelor's degree in 1964.
Don had never evinced interest in show business, and after leaving the club made no effort to return to acting. Instead he worked towards becoming a CPA, and eventually became Credit Manager for a large Orange County firm. In 1980 he learned that the Disney Studio was looking for him and other ex-mice to take part in the 25th Anniversary Reunion. Don appeared on the televised celebration wearing the long handlebar mustache that would become his lifelong trademark.
In the early nineties Don said goodbye to Southern California and the accounting world, and retired to Summerhaven, Arizona. This was a small hamlet, of about 100 year round residents, atop Mt. Lemmon, in the Catalina Mountains, just northeast of Tucson. Don purchased the Alpine Lodge, the village's largest building and employer, which he remodeled and ran for nearly ten years. The locals, an eclectic mix of mountain folk, retirees, ex-hippies, mysterious souls without last names, naturalists, and the like, began calling him "Daddy Don", for he was the "go-to" guy for solving community problems.
In June 2003, a wildfire swept the Catalina Mountains. The residents of Summerhaven and firefighters fought the blaze for weeks. Don coordinated supplies for the volunteers and relief efforts for those whose cabins were destroyed, putting folks up for free in his small lodge. But on June 22, 2003, the fire was whipped up by dry winds and suddenly swept through Summerhaven. Fortunately no one was hurt, but half the village, including Don's lodge, car, and home, were destroyed. Though the fire was soon extinguished, Don was left in a tricky position. He owned the now destroyed lodge, but not the land on which it stood. Indeed, his lease required him to keep paying rent, despite his loss. Don collected his insurance payment and exercised his lease option to buy the still smoldering land, then put it back on the market, and bought a home in Tucson. He told a reporter:
"It would have been too much of a physical and emotional effort to rebuild. I was thinking about going back to California and I realized I don't know anyone there anymore."
In April 2004, the American Red Cross honored Don with an award for his leadership in helping fight the fire and providing relief for those made homeless by it.