Kornmann then received a Bachelor of Arts degree from College of St. Thomas in 1959 and a Bachelor of Laws from Georgetown University Law Center in 1962.
He was in the South Dakota National Guard from 1963 to 1972 and the United States Army from 1968 to 1972.
Kornmann was an Executive secretary of South Dakota Democratic Party from 1963 to 1965.
On January 23, 1995, Kornmann was nominated by President Bill Clinton to a seat on the United States District Court for the District of South Dakota vacated by John Bailey Jones.
Kornmann was confirmed by the United States Senate on March 24, 1995, and received his commission the same day.