Stephenson was the Teessiders’ under-23s coach between 2017 and 2019 and is now the assistant manager at Kilmarnock
Hemming, 21, has spent time out on loan with Hartlepool and Blyth Spartans, and will now hope to gain some first-team experience in Scotland.
Kilmarnock were relegated from the SPL last season and will be looking to win promotion back to the top flight.
Hemming told the club’s website: “I was really pleased to get it over the line, this is a massive club. Hopefully I can help get us back to where we belong.
“I knew Paul from the ‘Boro academy and worked with him later when I went on loan to Blyth.
“My last loan spells really helped me mature a bit. I think coming here, I can kick on again.
“With the manager having been a goalkeeper himself then it’s good to have people here that can help me.”
Hemming is one of several young goalkeepers at Boro, including Sol Brynn, 20, and 21-year-old Brad James, who helped Hartlepool win promotion from the National League via the play-offs.
Boro boss Neil Warnock is keen for all three youngsters to leave on loan but is wary one may have to stay at Boro and act as a third-choice stopper.
The Teessiders have signed Joe Lumley from QPR, who is likely to be their No 1 option between the sticks after arriving on a free transfer.
Dejan Stojanovic remains under contract at the Riverside but is likely to depart following a loan spell at German club St Pauli.
The Bundesliga 2 club are keen to sign Stojanovic permanently but a deal will depend on finances after Boro spent a reported £1million on the player and offered him a contract until 2023.
It means Warnock is still looking to sign another keeper to challenge Lumley for the 2021/22 season.
Jordan Archer made six appearances for Boro last season after signing a short-term contract in January but hasn’t been offered a new deal as the club assess their options.