Secret Service agent Jack Ready, at front, keeps watch as President John F. Kennedy delivers a speech in Fort Worth on Nov. 22, 1963. (White House photo by Cecil Stoughton)

Jack Ready, a special agent in the U.S. Secret Service, thought he heard firecrackers as the presidential motorcade made its way past Dealey Plaza in Dallas on Nov. 22, 1963. He realized his mistake when someone said that the president had been shot.

Mr. Ready, a key member of the White House detail that guarded President John F. Kennedy on the day he was assassinated, died Feb. 24 in Fort Washington, Md. He was 86.

During a two-decade career with the Secret Service, Mr. Ready helped protect two presidents, an ex-president, a vice president, numerous visiting foreign dignitaries and Secretary of State Henry Kissinger.

The job demanded grueling hours and long stretches away from home during presidential trips — the most famous of which was to Dallas in 1963. Mr. Ready (pronounced “Reedy”) was assigned to the Secret Service car that followed the vehicle carrying Kennedy, first lady Jacqueline Kennedy, and Texas Gov. John B. Connally and his wife, Nellie Connally.

The motorcade was scheduled to travel from the airport, Love Field, to the Dallas Trade Mart, passing through Dealey Plaza. To the Secret Service agents, potential danger was always present. At Love Field, Mr. Ready and the others struggled to guard Kennedy as he engaged with the crowds that had come to see him.

“The crowd was going wild — people were moving closer and closer together, vying for the prime spots next to the fence,” former agent Gerald Blaine recalled in his memoir “The Kennedy Detail,” written with Lisa McCubbin. “In the agents’ minds, any one of those people could have a gun or a knife.”

When the motorcade departed, Mr. Ready was stationed on the follow-up car’s right-front running board. His position placed him opposite agent Clint Hill.

When the shots rang out, Hill rushed forward and — in a scene that would become iconic — leapt onto the president’s car in an effort to shield him and the first lady from the bullets. Mr. Ready, too, prepared to jump. But just as the third shot was fired, and as his driver veered to avoid hitting Hill, Mr. Ready was ordered to stay in place.

“Because of where he was positioned, he didn’t have a chance really to react or respond to the president,” Hill said in an interview.

“I know that it was devastating to Jack that he was unable to do anything,” Hill continued. “But he went on. . . . You could always rely on Jack.”

At Parkland Memorial Hospital, Mr. Ready manned the hallway where the first lady waited for news from the doctors. His eyes, Blaine wrote, were “so filled with pain it was almost unbearable to look at his face.”

In time, a presidential aide called on Mr. Ready for help.

“Jack,” he said, “I need you to find a priest.”

John David Ready was born Aug. 12, 1927, in Springfield, Mass. He joined the Navy at the end of World War II and served again at the beginning of the Korean War. In 1954, he received a bachelor’s degree in political science from American International College in Springfield.

He joined the Secret Service in 1960 and was assigned to the White House the next year. After serving Kennedy, he worked on President Lyndon B. Johnson’s detail and later, during Richard M. Nixon’s administration, for Vice President Spiro T. Agnew.

Mr. Ready then was assigned to protect Kissinger, who served as Nixon’s national security adviser and secretary of state. Mr. Ready accompanied Kissinger on his secret trip to China in 1971 in preparation for Nixon’s historic visit the following year.

“I had never met any Chinese Communists; neither, I must add, had the members of my Secret Service detail, who had not been told of my destination and who nearly had heart attacks at what they were witnessing,” Kissinger wrote in his memoir “White House Years.”

Mr. Ready and the other agent, Kissinger added, “did their duty by sticking grimly by my side wherever we were taken.” They followed him to meetings with Chinese Premier Zhou Enlai and “comported themselves,” Kissinger wrote, “as if they were a match for whatever 800 million Chinese might throw at them.”

Mr. Ready also handled security for Kissinger during meetings with the North Vietnamese preceding the 1973 Paris Peace Accords that helped end American involvement in the Vietnam War. He later served former president Nixon and his family at their home in San Clemente, Calif., as well as presidential nominees in the 1980 election, his daughter said, before his retirement in 1981.

Mr. Ready was a Fort Washington resident. His wife of 49 years, Paula Waterhouse Ready, died in 2013. Survivors include a daughter, Jane L. Ready of Alexandria; and a sister.

Jane Ready confirmed her father’s death and said the cause was complications from an infection.

She recalled that her father rarely spoke about his experiences in Dallas and that he revealed little about the leaders whom he protected and who counted on his discretion.

It was said that Mr. Ready had a close relationship with Kissinger and that the two men shared good humor in tense circumstances. Kent Jefferies, a younger Secret Service agent and friend of Mr. Ready’s, related one episode in a eulogy at his memorial service.

“I know you are brave and are good with a weapon,” Kissinger once told Mr. Ready. “What would you do if we were attacked by terrorists trying to kidnap me?”

“I have my instructions, sir,” Mr. Ready replied, knowing that the secretary would not mind his ribbing. “You are not to be taken alive.”