Sir Thomas Dalyell of the Binns, 11th Baronet, aka Tam Dalyell, was the longest serving MP until recently: from 1962 to 2005, at first for West Lothian, since 1983 he represented Linlithgow.
After his retirement - Member of Parliament for Swansea West since the 1964 - became "The Father of the House"
House of Commons Information Office: The Father of the House
However: Peter Tapsell
first entered Parliament in the 1959 general election, representing Nottingham West, and is the Conservatives' longest-serving MP albeit with a break in service (1964 to 1966). He is the only current MP of any party first elected in the 1950s, but the gap in his parliamentary service has thus far prevented him from being Father of the House.