She joins the trade body from the Electoral Commission where she was public affairs manager.
She has also had spells with the Royal College of Nursing and the Scottish Executive.
Keyes replaces Amy Hefford who is leaving to join a pharmaceutical company.
WSTA chief executive Jeremy Beadles said: “I am delighted to welcome Aileen to the team. She is an experienced public affairs manager with a track record of success and I am confident her campaigning skills will be a real asset.”
Beadles thanked Hefford for her contribution to the organisation and said she had led policy work in Scotland with “skill and determination”.