He volunteered for the United States Marine Corps a week after the Attack on Pearl Harbor, soon transferring to the U.S. Navy and receiving training at the Naval V12 program at the University of Washington.

[8] He fought with the Navy amphibious squad in the South Pacific Theater of World War II, and was injured in Saipan.

[8] While at the University of Washington Durkan met student Lorraine "Lolly" Noonan, whom he began a relationship with and later married in 1951.

[10][11] Durkan began his political career when he was elected the third vice president of King County Young Men's Democratic Club in November 1955.

[14] Durkan was elected to the Washington House of Representatives in 1956, defeating Republican William Crow to win the 31st district.

[7][15] He was a member of the Subcommittee on Ways and Means, which urged Governor Rosellini to veto Initiative 199 after it was approved by voters the previous year.

[19] He drafted legislation allowing the Seattle Center Monorail of the 1962 World's Fair to be owned an operated by the state instead of the city, which Rosellini was against.

[21] After reducing budgetary spending on funding for mentally handicapped people, he introduced legislation to close Northern State Hospital in Sedro-Woolley.