Is it considered workplace discrimination to be told continually to 'be happier'?

Yes, it is a form of discrimination, and it can transition into harassment depending on the frequency of the comments. I've represented a person with a similar "flat mood" related to diagnosed depression. Your case is more supportable than hers because you have chosen to be transparent in explaining to your "workplace" that your lack of facial expression is a symptom of your depression. Now, for the inevitable attorney-talk qualifiers: a disability must be accommodated only if doing so will result in the employee performing the "essential functions of the job." A related question is whether a "happy face" is a "bona fide job qualification" ["BFQ"] for your particular job description. This question may depend on whether you interact in a highly visible sales or public relations position, for example, or at a secluded office desk location with little customer/client contact. interaction. Assuming you are doing the job satisfactorily, and the problem is simply that people around you are uncomfortable with your flat or negative mood, that is not reason to deny you promotion or equal opportunity. I would compare the frequent encouragement to "look happy" to encouraging a person with a knee disability to walk without a limp. Your logical next step is to work with your human resources department to commence a formal "interactive process" that will marshall input from your psychiatrist describing work limitations. You employer should provide you with a job description to deliver to your doctor for that purpose. Then negotiate an accommodation with your employer that will include an explanation to co-workers and managers that they are not to badger your to improve your mood. Likewise, ask for a review of the promotion denials, and seek assurance the next promotion review will be non-discriminatory. Finally, depression is an illness, not a personality flaw. Your managers and co-workers obviously feel uncomfortable, perhaps even anxious, because they may be unfamiliar with the disease. Part of a compassionate solution is education. It may be an opportunity for the "workplace" you describe to be more accepting and aware. This approach involves something of a trade off -- your privacy in exchange for their education and adaptation. But you state you are already transparent about the condition, so it just may work.

Yes, but more information would be helpful so we can properly evaluate this.