The municipality says the card was reported stolen on Sept. 24, though officials believe it could have been taken several days before then.
Officials say the card contained the name, address, birth date/age, marital status and assessment information of clients enrolled in the region's Healthy Babies Healthy Children program between March 2010 and August 2011, along with some earlier dates.
They say the stolen information does not include social insurance numbers or health card numbers, but the region's associate medical officer of health says those affected should monitor their financial statements as a precaution.
Region Chair Emil Kolb has apologized and calls the theft an isolated incident, saying it's not "standard or acceptable" for the municipality to leave client information on unencrypted devices.
Provincial privacy commissioner Ann Cavoukian says the breach is being examined by her office, which has issued several orders stating that personal information must never be left unencrypted when stored on portable devices.
"I am astounded to learn that someone within Peel Public Health appears to have been using unencrypted mobile storage devices to store personal health information," Cavoukian said in a statement Monday.
Kolb said the municipality has launched a wide review in the aftermath of the robbery.
"As part of our investigation into this breach, we will be examining all of our privacy and protection protocols, and tightening controls on the information that has been entrusted to us," he said in a release.