Is Bartleby mentally ill?

The simple answer is that we don't know for sure. It would require Bartleby's examination by a trained professional to settle the matter once and for all. However, there seems little doubt that the poor, troubled clerk does seem to be suffering from some kind of mental illness, possibly depression. It's his complete apathy which points toward this—necessarily provisional—conclusion. Many people when they get depressed simply feel that they can't do anything; they remain trapped in a state of torpor, in some cases physically unable to move. That's the state that Bartleby appears to be in. He's not being willfully defiant in staying put; it's simply that he's been taken over by a sudden, deadening bout of lethargy, a common symptom of depression.