McCormick was imprisoned in 2008 for killing two young boys in a car crash while drink-driving.
The 29-year-old former Plymouth Argyle shot-stopper had been training with the League One club after his release from Leyhill Prison earlier this year.
He was sentenced to seven years and four months in 2008 after Arron Peak, 10, and his brother Ben, aged eight, died when the vehicle they were travelling in was hit by McCormick's Range Rover on the M6 in Staffordshire in June of that year. McCormick was twice the drink-driving limit.
Swindon interim chairman Jeremy Wray said in May he would resign if the decision to allow McCormick to train with his club damaged its reputation.
On Thursday, Swindon released a statement on their official website which read: "After careful and considered thought, Luke McCormick has decided to pursue footballing opportunities away from Swindon Town FC but remains eternally grateful for the support and friendship that the club has extended towards him in recent times.
"Following his release from prison, Swindon Town offered him the opportunity to train at the club as part of his rehabilitation.
"The club made it clear from the outset that a full time professional contract would not be on offer for some considerable time, if at all."
The statement from Swindon continued: "Now that the league season has got under way and with (manager) Paolo Di Canio wishing to focus all of his attention on his first team squad, the opportunities for Luke to continue training would be restricted to the youth and development squad.
"Having considered this as an option, along with embarking upon his community project, McCormick took the decision to explore other possible options and the club wishes him well with this endeavour."
McCormick added: "I shall be forever grateful to Swindon Town FC for holding out a hand of friendship and offering me an early opportunity to re-integrate into society, following my release from prison.
"My time at Swindon Town was an important stepping stone on the journey and I would now like to move forward in an attempt to re-establish a career in football, in addition to working within the community, alerting people as to the stupidity and dangers of drinking and driving."
McCormick was with Plymouth when the accident occurred over four years ago, and had been returning from former team-mate David Norris' wedding in Bolton.
Arron and Ben Peak were travelling to Silverstone racetrack for a dream day out with their father Phil and three friends when their Toyota Previa people carrier was hit by McCormick's vehicle at 5.44am on June 7.
The goalkeeper had been seen drinking beer and downing shots of Sambuca at the wedding reception.
Stoke Crown Court, where McCormick admitted causing death by dangerous driving and drink-driving, was told police investigators estimated his Range Rover was travelling at around 97mph moments before the crash.
Mr Peak, from Partington in Manchester, was at the wheel of the Toyota and suffered serious injuries that left him in a wheelchair.
Argyle terminated McCormick's contract by mutual consent after he was charged.
