Peel Region chair Emil Kolb said Wednesday in a statement that the senior was trapped in the elevator at the Malton Village Long Term from the evening of Dec. 23 until early Christmas morning.
Kolb says the woman had been out with her family on Dec. 23 and was dropped off at the front door that evening, but staff thought she was staying with family when she didn't return to her room.
It's believed she became trapped in the elevator between floors on her way to her room and it no one realized that she was missing until 24 hours later.
A search was begun and she was finally found in the malfunctioning elevator, was treated at the scene by paramedics and taken to hospital for evaluation.
Kolb says she returned to the centre on Christmas Day by dinnertime and although she is recovering well, she remains under close observation.
Kolb called the incident "very distressing" and said it was "very unhappy news on what should be one of the most joyous (days) of the year."
"My distress was certainly far less than that of the resident and her family," he added.
"We consider this an extremely serious failure," Kolb said.
The Ministry of Health and Long Term Care was advised of the incident and Kolb said regional officials are co-operating fully with the investigation.
He said action has already been taken to ensure residents on leave from the centre are accounted for on their return, including requiring that families and residents sign in and out of the centre.
"No resident or family should ever have to experience such an event again," Kolb said.