What do I have to prove to win a professional negligence claim against my accountant in Ontario?

You need to prove four things: your accountant owed you a duty of care, they breached the standard expected of a reasonably competent accountant, that breach caused your loss, and you suffered a quantifiable financial loss as a result. The duty is usually easy to establish once there's an engagement - the harder parts are breach and causation.

Breach is measured against what a reasonably skilled accountant would have done in the same circumstances, not perfection. Missing an obvious deduction, misapplying a clear rule, or failing to flag an evident risk can qualify; a defensible judgment call on an unsettled or ambiguous point usually does not, even if it turns out badly. Because this comparison requires knowledge of accounting practice, you'll typically need an expert accountant to testify about what the standard actually required.

Causation is often the real battle: you must show that, but for the negligent advice, you would have avoided the loss - not simply that a mistake was made somewhere in the file. Damages are then calculated based on your actual position versus where you would have been with competent advice, which can include penalties, interest, or lost opportunities, but not taxes you would have owed regardless.