If someone never signed a power of attorney and loses capacity, is court guardianship always necessary in Ontario?

Not always, though it's often needed. If someone loses capacity to manage their property without ever having signed a valid continuing power of attorney, and a capacity assessment finds them incapable of managing property, the Public Guardian and Trustee generally becomes their statutory guardian of property automatically, without anyone needing to go to court first — though a family member can then apply to replace the Public Guardian and Trustee in that role once appointed.

For personal care decisions, the picture is somewhat different: many day-to-day health care decisions can still be made under the Health Care Consent Act's own rules for substitute decision-makers, using a hierarchy of default decision-makers such as a spouse or children, without necessarily requiring a full court-appointed guardian of the person. A court application for guardianship becomes necessary mainly where there's a genuine dispute about who should act, where the statutory guardianship or default substitute decision-maker process doesn't adequately address the person's actual needs, or where a specific decision falls outside what those simpler processes can cover. Get legal advice on your relative's specific situation before assuming a full court application is the only option.