After serving in the U.S. Army from 1946–1947 and working as an architect in the Chicago office of Skidmore, Owings and Merrill from 1947–1951, Obata returned to St. Louis in 1951 to join the firm of Minoru Yamasaki, designer of the World Trade Center towers. Four years later, in 1955, he joined architects George Hellmuth and George Kassabaum in establishing the St. Louis-based architecture firm Hellmuth, Obata and Kassabaum.