He attended the University of Colorado at Boulder for two years, working as a reporter for the Boulder Daily Camera and contributing to the campus humor magazine, the yearbook and the campus newspaper. While still in high school, he worked as a cub reporter for the Grand Junction Daily Sentinel, covering courts, the high school, the mortuary and civic organizations. Trumbo graduated from Grand Junction High School. He was proud of his paternal ancestor, a Swiss immigrant named Jacob Trumbo, who settled in the colony of Virginia in 1736.
His family moved to Grand Junction in 1908. Trumbo was born in Montrose, Colorado, the son of Maud (née Tillery) and Orus Bonham Trumbo.