Conversation so far:
What work did he do on Doctor Who?
writing the second Doctor Who serial.
What contributions did he make?
The serial introduced the eponymous extraterrestrial villains that would quickly become the series' most popular and enduring monsters, and resulted in a major merchandising success for the BBC.
What was the name of these villains?
The Mutants.
Did he state how he came to create these characters?
unemployed, and with a young family to support.

Question: When was he hired to produce these characters for Doctor Who?
Self-contained rewrite: When was Terry Nation hired to produce eponymous extraterrestrial villains for Doctor Who?