Does operating under an incorrect or unregistered business name affect a director's personal liability in Ontario?

Potentially, yes. When a corporation carries on business under a name other than its exact corporate legal name, it must register that business name under Ontario's Business Names Act. If a corporation fails to register and then enters into a contract using the unregistered name, there can be consequences for enforceability and potentially for who bears liability under the contract.

More directly, if a director or officer enters into a contract on behalf of a corporation but does not make clear they are acting in a corporate capacity — for example, by signing without indicating the corporation's name — that individual may be personally bound by the contract on the basis that a counterparty reasonably believed they were dealing with the individual personally.

This risk arises most frequently in small businesses, where founders sometimes sign documents as their own name without specifying their corporate role, or where the corporation's registered name and operating name differ without proper registration. Contracts, invoices, and correspondence should always clearly identify the corporation by its legal name (or properly registered business name), and signatures should make the corporate capacity clear — for example, "XYZ Inc. per: [Name], Director." A corporate lawyer reviewing your standard contract templates can catch signature-block errors that create personal liability risk.