We meet people all the time who have a desire to mow their own lawns, but are no longer able to safely get into operator seat. Below are several options depending on disability and type of machine. As with any accommodation, safety should be the first concern. Do not implement any of these solutions if you cannot operate the machine safely. 

 
The lift shown above costs about $4000, and is attached to the mower so you can get on/off wherever the mower is at.

This lawn tractor has a seat that swivels 180 degrees, allowing access from the rear. Your wheelchair would need to be on an elevated platform to transfer straight across to the mower seat.

This picture shows an elevated wheelchair platform next to a zero-turn mower. And overhead grab bar helps the operator transfer from wheelchair seat to mower seat. The control lever between the wheelchair and mower seat has been modified so it can be removed during transfer, then re-attached to drive the mower.

This hydrostat lawn tractor has been outfitted with hand control levers and a seat that swivels 180 degrees. The operator owns a wheelchair with a seat elevator so he can transfer directly across from wheelchair to lawn tractor, then swivel the seat back to the forward position to drive as shown.

The picture above is a John Deere EZ Trak zero turn mower with an air-ride seat installed on superslides. The seat extends all the way out to the front of the platform so the operator can access the seat directly from his or her wheelchair. Once again, the wheelchair is up on a small platform so the transfer can be straight across.

Above is a Surehands wall-mounted sling lift with an extension that allows pivoting in two places. Something like this positioned in your garage could lift you from your wheelchair and swing you into place on your lawn mower.