Dunn began his career with
Christian Brothers College (CBC) of the St. Louis Municipal League, the city's de facto second division. CBC won the league title, but Dunn moved to the
Missouri Athletic Club for the 1915–1916 season. Dunn won a second Municipal League title that season before turning professional with the
Ben Millers of the
St. Louis Soccer League. Dunn played the 1916–1917 season with Ben Millers, winning another league title. At the end of the season, Dunn enlisted in the U.S. Navy for two years. When he returned to St. Louis in 1919, he re-signed with
Ben Millers becoming the team's starting center forward. Ben Millers won the 1919-1920 league title. In May 1920, Ben Millers won the
National Challenge Cup over
Fore River of the
Southern New England Soccer League. Dunn, captain for the game, scored the winning goal, twelve minutes into the second half for the 2–1 win. Ben Millers did not win another league title until the 1924–1925 season. However, that championship put them into the one-time American Professional Soccer Championship. In 1925, the
St. Louis Soccer League and
American Soccer League boycotted the
National Challenge Cup. In its place, they competed in what they called the American Professional Soccer Championship which pitted the champions of the two leagues. The
Ben Millers faced the
Boston Soccer Club for a two-game series. The first, played in St. Louis, went to Ben Millers, with Dunn scoring the game's lone goal. On the return game in Boston, the 'Wonder Workers' won 3–1, with Dunn again scoring. The final, and deciding, game took place two weeks later in St. Louis. The 'Wonder Workers' won the game and the title with a 3–2 victory. Dunn again scored in the loss. Ben Millers repeated as league champion in each of the next two seasons and finished runner-up to
Bethlehem Steel in the 1926
National Challenge Cup. Over his nine seasons with Ben Miller, Dunn scored fifty-three goals. ==Honors==