Mountjoy was born in Los Angeles, California and graduated from Monrovia-Arcadia-Duarte High School in 1950.[2] He joined the US Navy and served overseas during the Korean War. After his navy service, he worked briefly as an auto mechanic. He then started the Mountjoy Construction Company with his brother, Gordon.
A life member of the Veterans of Foreign Wars, Mountjoy was married to Earline Winnett until her death in 2009. They had two sons, Michael and Dennis Lee, and one daughter, Judy. Dennis was a member of the California State Assembly, having represented his father's old district from 2000 to 2006.
He ran for the U.S. Senate in 2006, winning the Republican nomination by default, as no other major Republican candidate had filed. On September 22, 2006, the Los Angeles Times reported that a statement in his website's biography, that he served in the Korean War aboard the USS Missouri, was incorrect; ship records later confirmed that he actually served aboard the USS Bremerton. Mountjoy said, "I think it was just something that somebody picked up, it didn’t come from me." However, the campaign of the Democratic incumbent, Dianne Feinstein, opined that the error raised enough doubts about Mountjoy's credibility that he was "not qualified to serve the people of California."[4] Feinstein defeated Mountjoy by a wide margin, 59% to 35%, in the November 7, 2006 general election.