Following graduation from Purdue, Lamb was accepted into the United States Navy Officer Candidate School. Upon completion of his training, he served 18 months on the attack cargo ship , and then moved to the Pentagon where he served in the audio/visual office of the Assistant Secretary of Defense for Public Affairs. Lamb took up this role midway through the Vietnam War and, in addition to handling queries from radio and television networks, he attended press briefings with Defense Secretary Robert McNamara. In July 1967, following riots in Detroit, Lamb was sent there and tasked with providing recordings of news conferences of Governor George W. Romney of Michigan for the White House Situation Room. He also served as a White House social aide to Lyndon B. Johnson, in which role he escorted Lady Bird Johnson down the aisle at the wedding of Chuck Robb and Lynda Johnson. He later recalled, "For five years after I got out of the Navy and went back part of the time to Indiana, the only thing I was known to have ever done in my life was to escort Mrs. Johnson down the aisle." Lamb spent a total of four years in the U.S. Navy and was a Lieutenant, junior grade, at the time that he left. He later said that his time in the U.S. Navy "was probably the most important thing [he has] ever done".