The 27-year-old hadn't played senior football with the Swans since Round 4 when he suffered his latest concussion setback.
News of McCartin's retirement, effective immediately, was revealed on Monday afternoon.
McCartin informed his Sydney teammates of his decision on Monday morning.
"Paddy has worked closely with our medical team and external specialists since he suffered his concussion in Round 4 against Port Adelaide," Gardiner said as part of a club statement on Monday afternoon.
"While he continues to work on his recovery, Paddy's decision is based on the recommendation of independent specialists that it is not in his best interests to continue to play.
"Paddy's story is a powerful one. He has shown remarkable drive and resilience to overcome considerable setbacks to play at the highest level and fulfil his potential.
"After joining the club's VFL program in 2021, Paddy worked his way onto the senior list in 2022 where he quickly proved his talent, developing into an elite intercepting defender.
"He quickly embraced our club just as we embraced him, and he has been a highly valued teammate and much-loved player. Paddy can be very proud of what he has achieved, and we are grateful for the contribution he has made to the Bloods.
"Paddy will continue to have our full support and we wish him well in his next chapter."
McCartin expressed his disappointment at not being able to continue playing, noting his decision comes with his long-term health in mind.
"I am obviously disappointed that I'm no longer able to continue to do what I love, but I have to be guided by the specialists and I know that this is the right decision for me," McCartin said as part of Monday's club release.
"I want to thank the Sydney Swans for the faith shown in me and the opportunity to pursue my dream of playing AFL football. It has been a privilege to play for this great club and I will remember this time fondly.
"I want to especially thank Lucy, my family, teammates, coaches and medical staff for their wonderful support through this challenging period – I'm very grateful."
Alongside younger brother, Tom, McCartin executed a career rejuvenation in the Harbour City before again succumbing to the dreaded effects of concussion.