To my knowledge, the longest-serving federal circuit court judge who is still hearing cases is James Browning of the Ninth Circuit, appointed by President Kennedy in 1961. President Johnson’s appointees who are still hearing cases include Wilfred Feinberg of the Second Circuit, Ruggero Aldisert of the Third Circuit, Myron Bright of the Eighth Circuit, and William Holloway of the Tenth Circuit. Am I missing any other Kennedy or Johnson appointees?
UPDATE: According to commenters, Judges Browning and Feinberg both recently retired. In both cases, their retirements are very well earned.