It's hard to know where "Clara" or Louise Coleman actually starts as a companion. She first appears as Oswin Oswald in Aug 2012, then pops up again at Xmas 2012 in Victorian London, before finally getting on board in March 2013.
For the record, Lis Sladen started in December 1973 and left in October 1976. Janet Fielding entered the Tardis in February 1981 before deciding it wasn't fun any more in February 1984, three years later.
Then there's Frazier Hines, who started in December 1966 and left (with Wendy Padbury and Patrick Troughton) in June 1969, clocking up an incredible 116 episodes! Hamish Wilson appeared as Jamie in 2 episodes in 1969's The Mind Robber.