Mertes attended grade school in Merrill and graduated from Gehlen High School in 1965. He completed college coursework from Sioux Empire College in Hawarden, Iowa, and earned his certification as an aviation electronic technician in Memphis, Tennessee.
He joined the US Navy in 1965 at age 17. After finishing boot camp in San Diego, he was stationed in Texas, Tennessee, California, and served aboard the aircraft carrier USS Ranger in the Tonkin Gulf and South China Sea until being honorably discharged from active duty as ATN3 in 1968.
Mertes is a 30-year member of the American Legion, a lifetime member and current Quartermaster of Monona County VFW Post 2694. He is also a longtime member of Communications Workers of America Western Iowa Local 7103 and served as its president for 15 years.
He worked for Northwestern Bell and its successors as a lineman, tester, and technician for 33 years, working in Spencer, Sioux City and Onawa.
After retiring from the phone company, Mertes continued to serve in a number of leadership roles, including: President of CWA Local 7103, Quartermaster of VFW Post 2694, Chair and Vice-Chair of the Monona County Democrats, the Monona County Compensation Board, the Iowa Judicial Review Board, and remains a contributor and supporter of many other local civic organizations.
House District 17 includes Onawa, Mapleton, Pisgah, Missouri Valley, Ida Grove, Holstein