Following graduation from Chapman in 1961, Trumbo was an assistant coach at his alma mater for a year, then became the head coach at nearby Garden Grove High School in 1962 for four years. In 1966, he became a college head coach and athletic director at Culver–Stockton College, an NAIA program in Canton, Missouri.
Trumbo moved back west to northern California in 1970 to Sonoma State in Rohnert Park as athletic director, and added basketball coaching duties after the Cossacks went 3–24 in 1972, winless in a dozen conference games. Under Trumbo, Sonoma State was 16–8 overall in 1973 with ten conference wins, and went 18–10 the following season. The basketball program was dropped in 1974 for financial reasons and Trumbo departed for nearby Santa Rosa Junior College and was the head coach for nine seasons, posting a 212–68 (.757) record with seven conference titles.[5]
Idaho
Moving up to Division I, Trumbo was hired at resurgent Idaho in April 1983, replacing Don Monson, a charismatic alumnus from Coeur d'Alene who departed after five seasons for Oregon in the Pac-10 Conference.[2][3] The Vandals had been a last place team in the Big Sky Conference for five straight seasons in the late 1970s, but rose to second in 1980 and then won consecutive conference titles (regular season and tournament) in 1981 and 1982. The latter finished the regular season at 24–2 with a #6 ranking in both national polls,[6] and advanced to the Sweet Sixteen in the NCAA tournament. The 1983 team slipped back slightly, but was 20–7 in the regular season and was invited to the NIT, a first for the Big Sky. In Monson's last four seasons, the best stretch in program history, Idaho was 54–2 (.964) at home, with a 43-game home winning streak; attendance had twice topped 11,000 in the Kibbie Dome during the 1983 season.
As an outsider following a hero, Trumbo recognized that his task in Moscow to continue the recent success would be difficult;[7][8] with less talent and experience, Idaho slipped back into the Big Sky cellar in 1984 and attendance plummeted.[9][10][11][12] His teams went 27–59 (.314) overall (9–33 (.214) in conference) and he was relieved of his duties after three seasons in March 1986,[3][13] succeeded by Tim Floyd,
an assistant under hall of fame head coach Don Haskins at Texas-El Paso.[14][15]