Protect your face, arms and hands from flying shards of glass. Wear eye goggles and a hat to protect your face. Wear a long sleeve shirt and leather work gloves to protect your arms and hands. Protect the work area with heavy tarps.
Secure the mirror to the wall with duct tape. Put duct tape in a few places on top of the mirror or sides. Use suction cups with handles on the mirror if it is a very large mirror. Have two helpers hold the handled suction cups to steady the mirror when it comes loose from the wall.
Find out where the original builder put the glue under the mirror. Press on the mirror to see if you can determine places where the mirror indents in, as there will be no glue in these spots. When you determine where the glue patches are, draw a circle on the mirror around the glue spots to help you determine where you will focus your removal efforts first.
Remove the mirror with heat. In some cases, the glue that is used to hold the mirror up will soften with heat. If this is the case, you can pull the mirror away from the wall without breaking it or destroying the wall. Apply heat to the mirror with a heat gun, focusing on the glue circles.
If heat does not work, use a long, flat putty knife or flat metal yardstick to slide all the way around the edge of the mirror. Cut through the glue adhesive as best you can. If that does not work, use a long piece of wire that is used to hang portraits to cut through the glue. Slide the wire down between the mirror and wall. In a saw-like motion, move the wire back and forth down the length of the mirror trying to cut through the glue. Have someone gently pry the mirror from the wall while you loosen and cut through the glue.
If all other methods fail, break the mirror up into pieces. Use duct tape to cover the mirror in large cross patterns. When you break the glass the duct tape will prevent glass from falling and shattering. Wear protective clothing and eyewear, tap the mirror with a hammer in several places to crack the glass but not hard enough to damage the wall behind. Carefully pull off pieces of mirror from the wall.
If there is adhesive still attached to the wall once the mirror is down, use an adhesive remover or peel carefully. If the wall has craters where the glue was, you may need to have the wall patched or just cover the area with another mirror.