In April 1946, Lammers was a witness at the Nuremberg tribunal. Starting in April 1949, he was tried in the Ministries Trial, one of the subsequent Nuremberg trials, and sentenced to 20 years in prison. The sentence was later commuted to 10 years by U.S. High Commissioner John J. McCloy, and, on 16 December 1951, he was released from Landsberg Prison. Lammers died on 4 January 1962 in Düsseldorf and was buried in Berchtesgaden in the same plot as his wife and daughter.