John J. Mertens was a member of the South Dakota House of Representatives and the South Dakota Senate.
Mertens was born on July 16, 1869, in Two Rivers, Wisconsin. He attended Hamline University and served in the United States Army, achieving the rank of captain.
Mertens was a member of the House of Representatives from 1909 to 1912 and of the Senate from 1921 to 1924. He was a Republican.