York Region Public Health would only say that the individual was fully vaccinated, but did not provide any other details in a phone interview with CBC News.
The Toronto Star reported Wednesday that the individual is a male under the age of 30 and that he had not travelled outside Canada recently.
The measles, mumps and rubella vaccine is generally about 95 per cent effective, but in some individuals it's effectiveness can decrease over time.
News of the confirmation comes days after York Region Public Health contacted 45 people who may have been exposed to the highly infectious virus at a doctor's office in Markham, Ont., last month.
It was not immediately clear if the confirmed case was related to that incident.
A Pickering-area mother whose infant son was possibly exposed at that same doctor's office published a pointed Facebook post Tuesday that has since been shared hundreds of thousands of times. In the post, she targets the anti-vaccination movement, saying it has possibly endangered her son's life.
Last week, a west-end Toronto daycare set up a temporary MMR vaccination clinic after it was revealed that an adult man who later diagnosed with measles had dropped off and picked up a child at the facility in the weeks prior.