Lyndon Baines Johnson became president after President John Fitzgerald Kennedy's assassination. Johnson had been Kennedy's vice president. He served out the remainder of Kennedy's term, won re-election in 1964 and retired from politics in 1969.
Lyndon Johnson was originally one of Kennedy's rivals. They competed for the Democratic nomination for president in 1960. Kennedy won a number of primaries, getting the people's support, but Johnson lined up support from the establishment without ever participating in a primary election. Kennedy won the nomination and took Johnson as his running mate to placate the more conservative Southern wing of the Democratic Party. Kennedy's presidency was short. After Kennedy was gunned down on Nov. 22, 1963, Johnson was sworn into office. According to Biography, Johnson's presidential legacy is mixed; though he passed stunning social legislation, including Medicare, he also escalated the disastrous war in Vietnam.