If your riding lawn mower won't start and you're sure the battery is the problem, it is possible to jump start the engine. The process isn't fool proof, however. The amperage of some car batteries may be too much for some older riding mowers and could cause serious damage to your lawn mower. You should only charge a 12V riding mower battery with a 12V car battery. If you are unsure that your machine can handle a jump from a car, use a portable charger or take it into the dealership.
Examine the dead battery to be certain it is the problem. Put the key in the riding mower's ignition and turn it. If the mower doesn't respond at all (no noise and the engine won't turn over), remove the key and pop the hood. Check that the battery connections are secure, and clean off any corrosion you find. Try and engage the engine again. If the lawn mower still fails to respond, the problem is likely your battery.
Drive your car to your riding mower. Make sure that they are close enough for the jumper cables to reach each machine's battery easily.
Open the hood of the car (and the battery cover if there is one).
Turn the car engine off.
Attach the positive (red) clamp of the jumper cable to the positive node (usually red or marked by a "+" sign) on your riding mower's battery.
Attach the other positive end of the jumper cable to the positive node of your car's battery.
Attach the negative (black) clamp of the jumper cable to the negative node (usually black or marked by a "-" sign) of your riding mower's battery.
Attach the other negative end of the jumper cable somewhere on the car's engine block away from the battery.
Start the car engine.
Wait a few minutes before you attempt to engage the riding mower's engine. If nothing happens, wait a few minutes more and try it again.
Remove the cables in the reverse order that you connected them.
Leave the riding mower engaged for at least 30 minutes.