Born in Washington, D.C., Johnson has an older sister Lynda Bird. Johnson's first name was originally spelled "Lucy"; she changed the spelling in her teens as a rebellion towards her parents. As her parents both had the initials LBJ, they named their two daughters to have these initials also. Although her father was a member of the Christian Church (Disciples of Christ), her mother was an Episcopalian, and she and her older sister, Lynda Bird, were raised as Episcopalians. Johnson converted to Roman Catholicism at the age of eighteen, when she requested and received conditional baptism. Johnson had been baptized with water and in the name of the Trinity at five months old by an Episcopal priest in Austin, Texas. Her rebaptism caused protests from leading figures in the Episcopal Church, which made headlines, as the Roman Catholic teaching does not require converts who are already baptized to receive baptism a second time.
She was sixteen when President John F. Kennedy was assassinated in Dallas on November 22, 1963. Johnson heard of the assassination while attending a Spanish class at the National Cathedral School. She was unaware whether her father had been injured as well but realized he had been sworn in as the 36th President of the United States when Secret Service agents showed up on her school campus a few hours later. She later attended Georgetown University School of Nursing and Health Studies but dropped out in 1966 as the school prohibited married students (Johnson married her first husband in August 1966).