Lem Johns, with dark hair, stands partially obscured behind Jacqueline Kennedy, right, as President Lyndon Johnson takes the oath of office on Air Force One in 1963. (Cecil Stoughton/AP)

Lem Johns, a Secret Service agent who guarded Vice President Lyndon B. Johnson during the Kennedy assassination, returned on Air Force One to Washington as Johnson was sworn in as president, and later headed Johnson’s presidential security detail, died Saturday at his home in Hoover, Ala. He was 88.

His death was announced by his grandson, Michael Johns, according to the Associated Press.

On Nov, 22, 1963, the day of John F. Kennedy’s assassination in Dallas, Mr. Johns was one of three Secret Service agents riding in a convertible behind Vice President Johnson and his wife, Lady Bird, in the presidential motorcade. Agent Jerry Kivett drove the agents’ car and monitored radio reports of the parade.

Suddenly, they heard the first two shots.

Though unsure whether he had heard gunshots, vehicle backfire or firecrackers, Mr. Johns jumped from the security car and made a dash for the car carrying Johnson.

However, after a third shot, the entire motorcade sped up. Mr. Johns, on foot, was left behind on the street.

Rufus Youngblood, the chief agent in the Johnson security detail, used his body to cover the vice president and Mrs. Johnson as their car followed the Kennedy vehicle and a Secret Service car to Parkland Hospital.

Amid the chaos, Mr. Johns climbed aboard a car carrying photographers, then moved to the sidecar of a police motorcycle en route to Parkland. He found Johnson and the other Secret Service agents at the hospital, waiting as surgeons worked on Kennedy. Texas Gov. John B. Connally also was wounded but survived.

When Kennedy died, an unmarked police cruiser took Johnson and agent Youngblood to Love Field where Air Force One waited to fly Johnson to Washington. Mr. Johns, who had been seeking a secure hospital exit for Johnson and Youngblood, had to leave for the airport in another police car. Once at the airport, Mr. Johns screened all those who boarded Air Force One while awaiting the arrival of first lady Jacqueline Kennedy and the casket carrying the president’s body. There were concerns about the possibility of an attempt on Johnson’s life.

Mr. Johns was among those gathered in the Air Force One stateroom when Johnson was sworn in as president. In White House photographer Cecil Stoughton’s photo — perhaps the most famous picture ever taken aboard Air Force One — Mr. Johns stands behind Mrs. Kennedy as Johnson takes the oath of office.

The Treasury Department, then in charge of the Secret Service, gave Mr. Johns an award for his actions on that day. He advanced to chief of the president’s detail in 1965, and assistant director of the Secret Service in overall charge of the protective forces in December 1967.

During the Vietnam War, the Secret Service found itself on the front lines of violent anti-Johnson demonstrations. During a 1966 presidential trip to Melbourne, Australia, protesters drenched Mr. Johns and Youngblood in paint intended for the president. Another Secret Service agent, Bob Taylor, had his foot run over while getting between demonstrators and the presidential car during the melee.

Mr. Johns left Washington during the Nixon presidency but returned to work as administrative chief and security director for Joseph A. Califano Jr., then secretary of health, education and welfare under President Jimmy Carter.

Thomas Lemuel Johns was born in Birmingham, Ala., on Dec. 11, 1925. Mr. Johns served in naval aviation during World War II and then worked as a Birmingham firefighter. He graduated from Howard College (now Samford University) near Birmingham and served as a criminal investigator for the Bureau of Alcohol, Tobacco and Firearms.

He joined the Secret Service in 1954 and served on President Dwight D. Eisenhower’s security detail during the last two years of the Eisenhower administration.

According to the New York Times, survivors include his wife, Nita; a son, Jeff; three grandchildren; and five great-grandchildren.

Three generations of the Johns family have served in the Secret Service. In addition to Mr. Johns, son Jeff was on President Ronald Reagan’s security detail and grandson Michael was on details for Presidents George W. Bush and Barack Obama through 2010.

In 1998, Justice and Treasury department lawyers were considering legislation to exempt Secret Service agents from subpoenas regarding personal and political information overhead during their protective duties.

Mr. Johns favored such a law and tried to clarify the types of information that agents could be called to provide.

“If an agent witnesses a criminal act,” he told Newsday, “I don’t see that we can say we refuse to testify. But if something is in the realm of a personal or professional confidence, that should be exempt.”

He added, “You don’t ever want the president to tell you when you’re leading him to the presidential limousine: ‘We’re going to be talking about some top-secret stuff today. So we’re going to take a cab.’ ”

CLARIFICATION: The caption with the photograph accompanying this story has been changed to indicate where Mr. Johns is standing. An earlier version accurately said he was behind Jacqueline Kennedy but did not specify where.