The 25-year-old McNamara was only a teenager when winning York's biggest handicap on Heartbreak City for Tony Martin.
He will be supporting and assisting Dale Gibson, the PJA's racing director, and will have increased involvement with apprentice and conditional jockeys.
"I feel the time is right to embrace a new challenge," said McNamara. "I have had some wonderful experiences throughout my career, riding 138 winners, the highlights of which were the Ebor in 2016 and the Group Three Sirenia Stakes in 2020.
"There were plenty of highs and lows along the way and I will be forever grateful to the trainers, owners, horses and fellow jockeys who have supported me throughout.
"A fantastic opportunity has presented itself and I am incredibly excited to be joining the PJA team as Racing Executive. Considering my experience of the unique challenges and difficulties facing jockeys, I feel I can bring distinct value to the role in future."
Gibson said: "We are delighted that Adam will join the PJA in the new year. His race-riding experience and knowledge are vital components to his new position.
"Adam's appointment will deliver extra capacity to the PJA and therefore enhance the service we provide to all Jockeys. The PJA team look forward to integrating Adam into his role in the coming weeks."