Brennan had a supporting role as Alf Simes in Those Calloways (1965), his first movie for Disney. Brennan was given a dual role as D.J. Mulrooney and Knobby in The Gnome-Mobile (1967) and played Renssaeler Bower in The One and Only, Genuine, Original Family Band (1968).
Brennan spent his last years mostly in retirement at his ranch in Moorpark in Ventura. He died of emphysema at the age of 80 in Oxnard, California. His remains were interred at San Fernando Mission Cemetery in Los Angeles.