Muhlmann (German spelling "Mühlmann") was born in Dortmund, Germany. After high school, he worked as a bricklayer and part-time soccer player. He played as a goalkeeper for Schalke 04 from 1962 to 1966, including the first ever Bundesliga (German soccer's top league) season in 1963–64. In 1968, he played soccer in the North American Soccer League (NASL) for the Kansas City Spurs.