After service in World War II during the next two years, Lemm served as an assistant coach at the University of Notre Dame under Hugh Devore in 1945.

After again spending a single season with the Cardinals, Lemm resigned on January 12, 1960, to accept an assistant coaching position with the Houston Oilers of the seminal American Football League.

Lemm was offered the position by his former coach John Breen, the Oilers' Director of Player Personnel, and proceeded to lead the team to nine straight victories.

The team then won its second straight title with a 10–3 win over the San Diego Chargers on December 24, 1961, and Lemm was named AFL Coach of the Year by both UPI and the Associated Press for his efforts.

After orally agreeing to a contract for the next season, Lemm instead resigned on February 22, 1962, to take the top spot with the Cardinals, citing the proximity of St. Louis to his home in Lake Bluff, Illinois.

Following a 44–0 loss to his former team in St. Louis on November 1, 1970, Lemm announced he would be retiring at the conclusion of the year, this time citing health issues.