After serving in the United States Navy in World War II, Kauffman worked as a pharmaceutical salesman until 1950, when he formed Marion Laboratories with a $5,000 investment, operating it initially out of the basement of his home. He reportedly chose to use his middle name rather than his last name in order to not appear to be a one-man operation.
Marion Laboratories had revenues of $930 million the year before it merged with Merrell Dow Pharmaceuticals (now part of Aventis) in 1989. The company sale made more than 300 millionaires. Kauffman founded the Ewing Marion Kauffman Foundation in the mid-1960s to provide cash grants to encourage entrepreneurship across America and improve education for children and youth, mostly in and around the Kansas City metropolitan area.
At the encouragement of his wife, Muriel, Kauffman bought the Kansas City Royals baseball team as an expansion franchise in 1968 and owned the team until his death in 1993. During his ownership the team won six division titles, two league championships, and a World Series Championship (in 1985).
In his will, Kauffman stipulated that proceeds from the sale of the Royals would go to Kansas City area charities. A month before his death, Royals Stadium was renamed Kauffman Stadium in his honor.