Keith Alexander featured in some of the productions of Gerry Anderson. In addition to voicing the character of John Tracy and serving as narrator for the 1968 film Thunderbird 6, Alexander voiced Sam Loover and numerous supporting characters in the TV series Joe 90. His other Anderson appearances are the 1969 film Doppelgänger and the TV series UFO as Lt. Keith Ford.
In The Secret Service series, he voiced the following characters:
- 1st Guard - To Catch A Spy.
- 1st Native - Errand Of Mercy.
- 3rd Native - Errand Of Mercy.
- 3rd Technician (Port Trennick) - A Question Of Miracles.
- Agent - Last Train To Bufflers Halt.
- Agent Blake - Recurring Role.
- Ambassadors Aide - A Case For The Bishop.
- Archdeacon - School For Spies.
- Assassin - May-Day, May-Day!.
- Brooks - A Question Of Miracles.
- Brother Thomas - School For Spies.
- Burrows - The Cure.
- Commentator - May-Day, May-Day!.
- Co-Pilot - May-Day, May-Day!.
- Dave Marden - The Cure.
- Doctor - A Question Of Miracles.
- Engineer Hartley - A Question Of Miracles.
- French General - Recall To Service.
- General - Errand Of Mercy.
- General Brompton - Hole In One.
- Joe - May-Day, May-Day!.
- Kroner - The Deadly Whisper.
- Lady Martha Hazlewood - More Haste Less Speed.
- London Airport Controller - A Case For The Bishop.
- Officer - Errand Of Mercy.
- Operator - Hole In One.
- Professor Graham - Recall To Service.
- Professor Soames - The Deadly Whisper .
- Read - Last Train To Bufflers Halt.
- Saunders - A Case For The Bishop.
- Sir Humphrey Burton - To Catch A Spy.
- Signalman - Last Train To Bufflers Halt.