Susan possessed a TARDIS key in addition to the Doctor's own.
When on Quinnis, Meedla stole Susan's TARDIS key, bargaining its return for passage off the planet. When Meedla later perished, Susan reclaimed the key from the corpse. (AUDIO: Quinnis)
Susan's key was able to change shape and form, much like the Doctor's TARDIS did. In Berlin in January 1933, her key was hexagonal shaped, and she was relieved it wasn't stolen when she was kidnapped by Pollitt. (AUDIO: The Alchemists)
Susan took her key with her to Coal Hill School so that she could later use it to unlock the TARDIS upon returning to 76 Totter's Lane. (PROSE: The Rag & Bone Man's Story)
According to one account, Susan wore her key around her neck. She lost it when she became injured in a car crash on Barnes Common; the Doctor took her back to the TARDIS, but returned to the crash site to retrieve the key, which had been found by Ian Chesterton. When the Doctor asked for it back, claiming it was his, Ian knew he was lying, as Barbara Wright had told him it was Susan's. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)
When the Daleks ordered one of the TARDIS travellers to return to the ship and retrieve the anti-radiation drug, Ian Chesterton volunteered and asked for Susan's key but she refused on the basis that there was a security system that Ian would not know how to navigate around.
At the onset of the Thal-Dalek battle, the First Doctor destroyed this key, using its metallic composition to destroy a static electricity junction box and one of the Dalek City's videoscopes. He promised his granddaughter that he could make another one at a later date. (TV: "The Ordeal")