Lem Johns, a Secret Service agent who guarded Vice President Lyndon B. Johnson when President John F. Kennedy was assassinated in Dallas and who became a high-ranking Secret Service official during Johnson’s administration, died on Saturday at his home in Hoover, Ala. He was 88.

His grandson Michael Johns announced the death. Mr. Johns was the first of three generations in his family to serve as presidential Secret Service agents.

Mr. Johns was among three Secret Service agents riding directly behind the convertible carrying Mr. Johnson and his wife, Lady Bird, as the Kennedy motorcade wound through downtown Dallas on Nov. 22, 1963.

“I heard two ‘shots,’ not knowing whether they were firecrackers, backfire, or gunshots,” he wrote in a report to the chief of the Secret Service a week after the assassination. “I jumped from the security car and started running for the vice president’s car.”