Please bear in mind that I am not a lawyer. This article is not intended to give legal advice or contradict legal advice you might have received.
If you're having trouble dealing with your depression and it's affecting your work, it may be time to ask your employer for accommodations under the ADA. While disclosing your condition can be a scary step to take, chances are that it's better than being fired for poor job performance.
The Americans with Disabilities Act does cover disability due to mental illness. However, this only applies if treatment is not successful. Several Supreme Court rulings essentially said that if you are not limited in "major life activity" when you are under treatment, the disability for all intents and purposes doesn't exist, and you are not entitled to accommodation. Major life activities are "learning, thinking, concentrating, interacting with others, caring for oneself, speaking, performing manual tasks, or working."
If you are not experiencing complete relief from depression symptoms with treatment, the ADA may protect you in terms of requiring employers to make accommodations to/for your symptoms and treatment. The goal of these types of accommodations is to allow you, as someone with a disability, to keep working and contributing to society. The ADA doesn't unilaterally obligate the company to grant all requests. The accommodations must be reasonable, and cannot cause the company undue hardship. Make sure that you are asking for something your company can provide without too much cost or inconvenience.
Examples of possible accommodations:
Change of duties to ones that require less attention to detail.
Longer lunch or leave early/come in late for doctor or therapy appointment (you should offer to make up the time).
Managing side effects of medications.
Moving or modifying workstation to minimize noise and interruptions.
If you are more depressed in the morning, ask to change your hours to come in later and leave later or the reverse if you are more depressed as the day goes on.
Job-sharing or flexible hours.
Your chances of being accommodated are better if you go into your meeting with concrete suggestions. Your employer may be unfamiliar with the symptoms of depression. They might be perfectly willing to accommodate you if you explain why these accommodations will help you to do your job.
When (and how) should you ask for accommodations?
If you are struggling and you know that your work performance is being affected, you want to ask for these accommodations sooner rather than later. It's too late to ask when you are being fired if your employer is unaware that you are suffering from depression. If financially feasible, you should consult with an employment lawyer first. You definitely want to put your request in writing, even if it's just for the purposes of having something written to go over in your meeting. After you meet with your employer or human resources department, follow up with an email or memo summarizing your meeting.
Remember to look at the situation from your employer's point of view, especially if you work for a small company. Depression is an invisible disability. You're not in a wheelchair. Unless your employer knows your situation well, you might be seen as someone who's trying to game the system. Be businesslike and be prepared to work with your employer. Don't go into the negotiations with an attitude that says, "You have to accommodate me because it's the law." It may be true, but it doesn't make you seem committed to making things work.
How the ADA Applies to People with Psychiatric Disabilities
Disability and Business Technical Assistance Center
Consumer Website for Handling Your Mental Illness at Work and School