Can an insurer deny my claim just because I reported it late in Ontario?

Not automatically. Most policies require prompt notice of a loss or occurrence, and late notice can be a real problem, but Ontario law generally looks at whether the delay actually prejudiced the insurer's ability to investigate the claim or defend it properly, rather than treating any lateness as fatal on its own. An insurer that can show the delay genuinely harmed its position - evidence disappeared, witnesses became unavailable, or an opportunity to control the defence early was lost - has a much stronger basis to deny coverage than one relying only on the fact that notice wasn't immediate.

If you're facing a late-notice denial, it's worth asking what the insurer can actually point to as real prejudice, rather than accepting a blanket denial at face value. The specific wording of your policy's notice clause also matters, since some are stricter than others about what counts as timely and what the consequence of missing it is. Given how fact-specific this is, and how easily an insurer's prejudice argument can be tested, a late-notice denial is one of the more worthwhile coverage disputes to have reviewed rather than simply accepted.