Their 340-day mission aboard the International Space Station (ISS) is twice the length of a normal stay.
Their extended tour is part of an effort to study the effects of long-duration spaceflight on the body.
But scientists will gain further insights by comparing Scott Kelly with his Earthbound identical twin, Mark.
A Soyuz capsule carrying Kelly, Kornienko and Russian crew member Sergey Volkov parachuted down on to a steppe in
Kazakhstan at 10:26 local time (04:26 GMT).
The mission is a record for the ISS and will give Cmdr Kelly a tally of 520 cumulative days in space, over four flights.
Cmdr Kelly has also returned rocket (salad) seeds that were taken up to the orbiting outpost by UK astronaut Tim Peake in December.
They will be distributed to schools to allow pupils to compare their growth with rocket plants that have stayed on Earth.