Kellerman joined the Secret Service in Detroit just before Christmas 1941, transferring temporarily to the White House detail in March 1942 and permanently one month later.

Like all Secret Service agents assigned to protect the President of the United States, Kellerman was trained to use his own body as a shield, taking a bullet if necessary in the line of duty.

[citation needed] Kellerman along with Secret Service agents William Greer, Clint Hill, and Rufus Youngblood, provided testimony to the Warren Commission in Washington, D.C., on March 9, 1964.

"[3] He further testified to the Warren Commission that he first heard what sounded like a "pop" or a "firecracker" somewhere to his right and about five seconds later the assassination then ended in a "flurry of shells" coming into the limousine.

[10] However, in 1994, Vanity Fair published an article by Anthony Summers and Robbyn Swan in which they quoted Kellerman's widow, June, as stating he "accepted that there was a conspiracy.