Probably held onto the wall with mastic. Pry it off, more than like will break into pcs.

I pulled my mirror with glass handling suction cups. Maybe you can rent some. It's exciting, believe me.

Make sure you tape it across the surface to help catch pieces when it breaks.

Cover the entire mirror with heavy tape or special adhesive plastic they make to protect you from the broken glass.

I had a glass company do it. Mine was about 3' x 4'. He got these two "giant suction cups" and it took him less than five minutes to remove it in one piece.

It cost about $75 and was well worth it. I asked him he gave it to Habitat for Humanity or something similar and he said no. Not a bad $75 for five minutes of work + drive time.

Alternative.

After taping the face. get a piece of wire, piano wire, etc. Thin and strong.

Put it at the top of the mirror, between the mirror and the wall. Pulling left and right, you can "saw" the wire down gap between the mirror and the wall, cutting any adhesive.

When you get a bit down, secure the top of the mirror to the wall so it doesn't flop over when you get to the bottom.

I too hired a glass company. Got two prices 135 and 75 It seemed so simple for him but it was worth it b/c I didn't know if there was liquid nails holding it. After you get hurt doing something like that there's no going back for a "do over."

I called the glass company and they did it for $125 with the big suction cups. I would never ever consider removing a large mirror. Too much to go wrong.

I did remove the mastic myself - kind of fun.