It is clear that a terminated employee has an obligation to mitigate to assert a common law lost income claim, apart from the statutory sums which require no mitigation. This is one component of a lawsuit that is in the complete control of the plaintiff and one which should not present any issues to a well-prepared plaintiff.

Typically, it is a tough road to hoe for the employer to win such a defence as failure to mitigate, given the onus and the qualitative nature of the burden, particularly in that it is a double-barreled onus.

Cases that have succeeded in the plea are few. The decisions of Mifsud and Evans have created a new defence which must be examined closely, given the impact of such a submission. This type of defence, however, still remains a difficult one as the basic concept is counter-intuitive. The exceptions, which are driven by the specific facts and individual context, to Mifsud and Evans are wide enough for a front end loader to delicately navigate through the mine fields.

The Onus

The case law states without exception that the onus to prove a failure to mitigate rests upon the employer as set out in Red Deer College v Michaels in 1975 by the Supreme Court of Canada. It is oft repeated. The late Chief Justice Laskin described the burden on the employer in these words:

In the ordinary course of litigation respecting wrongful dismissal, a plaintiff, in offering proof of damages, would lead evidence respecting the loss he claims to have suffered by reason of the dismis­sal. He may have obtained other employment at a lesser or greater remuneration than before and this fact would have a bearing on his damages. He may not have obtained other employment, and the question whether he has stood idly or unreasonably by, or has tried without success to obtain other employment would be part of the case on damages. If it is the defendant's position that the plaintiff could reasonably have avoided some part of the loss claimed, it is for the defendant to carry the burden of that issue, subject to the defendant being content to allow the matter to be disposed of on the trial judge's assessment of the plaintiff's evidence on avoidable consequences. This is the way I read what is said on the matter in such leading textbooks on the subject as Cheshire and Fifoot's, Law of Contract, 8th ed. (1972), at p. 599, and Corbin, Contracts, vol. 5 (1964), at p. 248. The matter is put as follows in two passages from Williston on Contracts, vol. 11, 3rd ed. (1968), at pp. 302 and 312:

The rule of avoidable consequences here finds fre­quent application. The consequence of this injury is the failure of the employee to receive the pay which he was promised but, on the other hand, his time is left at his own disposal. If the employee unavoidably remains idle, the loss of his pay is actually suffered without deduction. If, however, the employee can obtain other employment,

...

It seems to be the generally accepted rule that the burden of proof is upon the defendant to show that the plaintiff either found, or, by the exercise of proper industry in the search, could have procured other employment of an approximately similar kind reasonably adapted to his abilities, and that in absence of such proof the plaintiff is entitled to recover the salary fixed by the contract.

Cheshire and Fifoot, supra, expressed the position more tersely as follows:

But the burden which lies on the defendant of proving that the plaintiff has failed in his duty of mitigation is by no means a light one, for this is a case where a party already in breach of contract demands positive action from one who is often innocent of blame.

The Qualitative Standard

As noted above, the burden is “by no means a light one”. As was stated by the British Columbia Supreme Court in Smith v Pacific National Exhibition, “it is often a heavy burden to satisfy”. ¹ Burnyeat J. described this as a “heavy onus” .

The Supreme Court of Evans in 2008 affirmed that the obligation to prove a failure to mitigate is a burden which is “onerous” in the dissent, given that it is the defaulting party which requires affirmative action from the victim. The plea must meet a “relatively high standard of proof”. ²

As noted by the Ontario Court of Appeal in its 2017 decision:

 there "is no magic formula that an employee must follow when making reasonable efforts to obtain other employment, and thereby mitigate his or her loss”.