Gerald Ford was the only person to be vice president and president of US without being elected for it

Gerald Rudolph Ford, Jr. (born Leslie Lynch King, Jr.) (July 14, 1913 – December 26, 2006) was the 38th President of the United States, serving from 1974 to 1977, and the 40th Vice President of the United States serving from 1973 to 1974. He was the first person appointed to the vice-presidency under the terms of the 25th Amendment, and became President upon Richard Nixon's resignation at noon on August 9, 1974. Ford was the fifth U.S. President never to have been elected to that position, and the only one to have held both the office of Vice-President and the office of President while never having been elected to either. He was also the longest-lived president in U.S. history, dying at the age of 93 (when six weeks older than Ronald Reagan).

http://en.wikipedia.org/wiki/Gerald_ford