Martenson first became involved in sandlot baseball and played for a team called the Triangles in 1929. He was an outstanding pitcher and took his talent to the Jamestown Chair team which was managed by Joe Nagle, who is a Chautauqua Sports Hall of Fame inductee.
In 1941 Martenson bought a restaurant, Harry's Lunch, on Forest Avenue and baseball was always the topic of conversation there. In 1964 Martenson and some other players formed a team to play the Warren Oldtimers which led to the formation of the Jamestown and Area Oldtimers team. It was formed by Martenson, Nagle and Morrie Mistretta, another 1987 CSHOF inductee. The Oldtimers' organization continued to grow and now plays a full schedule, each summer including road games in Ohio and Pennsylvania. Martenson was named the organization's secretary-treasurer in 1964 and held the position for many years.