McKinnon was born 1906 in Dallas, Texas to Dr. John and Tennie McKinnon. He was 12 and the only child when his father died and his mother, a nurse, raised him. He graduated from Palo Alto High School, Palo Alto, California. McKinnon attended Stanford University in 1924, attended the University of Geneva in 1930, and received a BA from University of Redlands in 1930.

McKinnon and his wife Lucille had two sons, Dan and Mike, and a daughter Connie.

McKinnon was a journalist and owned newspapers in Texas and California. He started local "throwaway" newspapers and sold ads to local merchants.

He was the only one to start a daily newspaper during World War II, the San Diego Daily Journal in 1944, by stringing together several throwaways. The Journal competited with the much-larger San Diego Tribune. This was possible because the Democratic Roosevelt administration allocated newsprint, then under ration, to the Journal to compete with the Republican Tribune. Other Journal alumni included Tribune editor and columnist Neil Morgan and Congressman Lionel Van Deerlin. Van Deerlin recalled:

He was one of the guys - very different from your standard publisher. At the company picnics, he would be pitching softballs. He would come through the newsroom and he would know the names of your kids and which ones had been sick. McKinnon sold the paper in 1947, before running for Congress, and it was eventually absorbed into the Tribune.

Time magazine described McKinnon as "a jockey-sized little fireball with unruly black hair and bounding energy".

McKinnon was first elected to the House of Representatives in 1948, defeating Republican incumbent Charles K. Fletcher, founder of Home Federal Savings and Loan. During the campaign McKinnon hogged the stage for 15 minutes keeping President Truman from speaking to the crowd waiting for him. Later in Washington, McKinnon apologized to Truman, and Truman glared back at McKinnon, then said :Well, you got elected, didn't you? That's the only thing that matters".

McKinnon served until 1953. In Congress he fought for water projects for San Diego. Gordon Luce, then the California Republican Party Chairman, said that he was a bright and hard-working leader that even political opponents respected. In 1952, instead of running for reelection, he ran for the Democratic nomination for Senator against better-known Senator Bill Knowland and lost. Since the Congressional districts were redrawn in 1952, McKinnon was the last Representative to serve all of San Diego County. McKinnon was a delegate to Democratic National Conventions in 1952 and 1956.

McKinnon would rather be known as a journalist than a politician. His other ventures included the La Jolla Light, Coronado Journal, and radio station KSDJ (now KCBQ).

McKinnon died 2001 in La Jolla, California.