My 20 hp MTD riding mower will not shut off even when you get off the mover or turn the key to the off position. Could you provide me with a solution to the issue

The kill wire most likely has a break in it. It's usually grounded to stop the engine.

Hi,
Most but not all MTD's use a yellow wire for ignition ground. It could be broken, unplugged or the ignition switch may not be providing a ground.
Peace,
Clarke