Left-back Wallace has joined on a two-year deal following the expiry of his contract with Rangers, while goalkeeper Kelly has moved from Livingston on a four-year agreement.
Wallace, 31, spent eight years at Ibrox and will link up again with former Rangers manager Warburton, who is now in charge of QPR. Warburton appointed him as captain during their time in Govan.
“I am delighted to be here and really looking forward to the challenge ahead. This is an exciting time to play for a top football club,” Wallace said on the club website.
“I have had 15 seasons in Scotland and now is time for a fresh challenge - and a very exciting challenge at that.”
Warburton said: “He is a natural leader and did a fantastic job for me while I was up there. He governed the dressing room and set the standards in the right way.
“The last 18 months at Rangers were difficult for him. I have no doubt he will come down here absolutely determined to prove many people wrong.”
Kelly, who began his career at Rangers, had invoked a release clause at Livingston, allowing him to leave for an undisclosed fee, believed to be £50,000, despite having one year left on his contract.
The 23-year-old Scotland squad goalkeeper said: “I felt now was the right time for me to try something new and QPR gave me that opportunity.
“Knowing the manager and how his teams play, it was something I couldn’t say no to. How the gaffer conducts himself is brilliant and that was a really attractive part of the move.”
Kelly kept 17 clean sheets for the Ladbrokes Premiership club after leaving Ibrox last summer, having previously helped Livingston to the League One title on loan.
Warburton said: “Liam is outstanding with his feet, he has great distribution and he is a very good shot stopper.”