Does a medical diagnosis of anxiety and depression, requiring both medication and medical leave from work, constitute a short-term disability? I've been on leave from work since January 17, but my employer's insurance policy only talks about long-term disability, and nothing regarding mental illness.

If you are medically disabled from working, then you likely do qualify for benefits. However, your employer would need to have STD benefits, also sometimes called Weekly Indemnity benefit, for you to be eligible for STD.
If you don't have STD through your employer, you can try to apply for Employment Insurance Sick Benefit through the federal government and that should help until your LTD benefits start.
If you have any further questions, or run into any issues with STD or LTD benefits, give us a call immediately and we'll help- our contact information is below.