By accident the key of the riding lawn mower was left in start position and drained the battery. car and mower battery are both 12V. Could I use the car battery to jump start the lawmower? Do i make the connections first, then start the car, then try to start the lawn mower?
My personal thought is that I don’t jump a vehicle with another vehicle or even a lawn mower because of the expensive electrical damage that can be done . You can buy a battery charger for not much money and you will probably need it again . Or you could just take the battery out and let Auto Zone charge it for you. Plus they have portable battery packs that are not expensive .
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if you really want to be safe unbolt both car battery terminals and attach jumpers to car battery and use it to jump mower. though i think if you have enough skill to use jumper cables than you dont need to take this step.
Battery chargers are really useful to have around the house for like $40-50 .
I jump started my riding mower which had a 12 volt battery from my Ford Maverick which also had a 12 volt battery. In fact, the battery in my mower was the same size as the battery in the Maverick. When the mower battery was shot, I bought a new battery for the Maverick and put the Maverick battery in the mower. I think my rider had a 15 hp engine and I never understood why it didn’t have an emergency rope start provision.
I also used to do.the opposite and jump.start my 1950 Chevrolet pickup from the mower. The battery in the pickup.was under the passenger side floor. You lifted up the floormat and removed an access plate to get to the battery. It was easy to maneuver the mower up to the right side of the truck. The truck had a 6 volt system and the 12 volt battery in the mower would really crank up that engine. T
This was over 40 years ago that I lived in the country on five acres. I don’t jump.start vehicles with today’s sensitive electronics.
I also agree that a small battery charger is a worthwhile $50 purchase.
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My old rider battery was dead this spring, jumped it from my minivan with standard jumper cables. No problems whatsoever. Connect as you would if you were jumping a car, turn on the car, then start the mower (note, because of where the safety switch was located on that mower and the battery being under the seat, I did have to temporarily defeat it while jumping). Sold it for $75 before we moved, considering it was gifted to me in not the greatest condition, I was pretty happy with it.
I’ve jump started my mower from a car battery, but just left the car engine off. The mower doesn’t draw that much from the car battery.
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But you are not on the web asking how to do the jump start thing.
I usually wind up jump starting my mower with my car about once every other season. It’ll start all summer, then it’ll be dead when I try to mulch leaves in the fall.
If I had that problem I’d remove the lawn mower battery & use a battery charger to charge it overnight on the work bench, and not involve the car.
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The overnight thing doesn’t work for impatient folk like myself!
I tend to not plan ahead. It’s a self admitted fault. I generally don’t say, “I’m going to mulch leaves tomorrow”. It’s more of an “I’m tired of raking these darn things up in piles and burning them” moment. “Darn mower won’t start!”
But your idea is great for the planners of the world.
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Remember I spent a week trying to remove a plug from a 50 year old rusty pipe, when I needed neither the pipe nor the plug removed from it; did it only to see if I could. The only conclusion: I couldn’t. … lol … as far as having get-up-and-go when some job or the other needs to be done, that’s what procrastination is for 
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I had always heard the maxim " Don’t put off untill tomorrow what you can do today" but my grandfather told me, " What you put off until tomorrow someone else might come along and do."
He was one of the happiest men I ever knew.
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I’m an anti-procrastinator. An anticrastintor? No, that sounds satanic.
At any rate, I tend to barrel ahead full force. Which can cause issues as well, but is a positive thing in some circumstances. The leaves will be mulched! Maybe the car will start too…
Planning ahead…not so good with that.
Throw the pipe out back in the pile of printers! 
I think my problem was watching “Leave It To Beaver” as a kid. I got all my personal philosophy from that show. In one episode the dad asks why Beaver is always late doing his homework? Beaver: “Because if I do it early, I might get run over by a truck, and then I’d have wasted all that time doing homework” … lol .
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Well my philosophy that never failed me was the amps are irrelevant, don’t need to start the car just jump it what I might do, right or wrong. The philosophy is the item will only take the amps needed. Failed me once using a 12 volt switching transformer in a 12 volt disc drive I think, fried that puppy in no time. Of course a trickle charge or new battery would be the logical thing to do, safety glasses, do the car battery last and if you survive let us know 
I’ve watched my dad do this once with our '90 Mazda Protoge and there were no problems. The battery on the lawn tractor was recharged using an actual battery charger later on but it got the mower going. There’s now a couple of jump-starters for this sort of thing in the garage.
Heck, I’ve jump started the lawn tractor with every battery operated conveyance I have. Used whatever was closest to it at the time. Car, motorcycle, atv, boat, loose battery. Grass isn’t going to wait for the mower to be ready nor am I. Now if I could just learn to turn off the key when I am done mowing…hasn’t happened in a while now but the odds are stacking up…
I’ve used a car to jump start a motorcycle. If they’re both 12 volt batteries, I’m pretty sure it’s safe.
My Dad’s 1939 Chevrolet had an emergency hand crank and I have seen him use the crank to start the engine. J.C. Whitney had a rope start kit for Volkswagen Beetles to start up the engine if the battery could not turn the engine over. Yet, my 15 hp lawn tractor that I owned years ago could only be started with the battery. It had a generator which served as starter to turn the engine over.
My guess is that the jumper cables manufacturers association lobbied the engine manufacturers from automobiles to lawnmowers not to provide alternative means of starting so that more jumper cables could be sold.