The actress who played the Doctor's grand-daughter, Carole Ann Ford, left the programme about a year after it started.
The character of Susan as described to her would have had telepathic communication with her grand-father and be rather an Avengers style character ( itself a great leap forward for female TV characters in the early 1960's ). However, by time the character reached the screen, she was a bit of a screamer, and Carole wasn't too happy. She was also in her 20's playing a teenager.
In recent years Carole has acted as a voice coach, particularly with politicians.
Carole returned to Doctor Who as Susan for the 20th anniversary scecial "The Five Doctors" with another actor playing the first Doctor ( actor William Hartnell died in 1975 ).
Here are details of the last original story Susan appeared in...
http://www.bbc.co.uk/doctorwho/classic/episode guide/dalekinvasion/detail.shtml