McMillan lost his British title to Paul Ingle in January and after four months of deliberation has decided to pursue a career on the other side of the ropes.
"I didn't want to rush into a decision," McMillan, 31, said. "I wanted to think it through and weigh up everything and now I feel it's time for me to call it a day and move on.
"I couldn't get motivated for the last fight against Ingle. Once you've fought at world level it's hard to get yourself motivated for other fights against some of the young, hungry guys who want to get your scalp on their record as a former world champion."
McMillan, who traded on speed and pure boxing skills, produced his most dazzling performance at the right time when he claimed the WBO crown from Italy's Maurizio Stecca, a former Olympic gold medallist, in May 1992.
The future then looked rosy for McMillan, but his career was cruelly turned on its head in his first WBO defence against Colombia's Ruben Palacio.
The Barking stylist sustained a dislocated shoulder which forced the referee to stop the fight in the eighth round. McMillan never really recovered from that setback. After an operation and 13 months out of the ring he was outpointed by Steve Robinson, the new holder of his former title.
He regained the British title 11 months ago by outpointing Doncaster's Jonjo Irwin in a thriller at Dagenham but the big chance never came his way again.
McMillan added: "I always knew it would be a hard route back after holding the world title. I'm sad in a way that I never realised my full potential because the injury came along and was a problem for the next couple of years.
"When I beat Stecca I felt at the time that there was still more to come, but everything came to halt after the shoulder trouble just at the time I thought I was reaching the highest level. I made a costly miscalculation against Ingle and I'm more annoyed with myself than anything else.
"I didn't think he was so fit and mentally prepared as he was and I tried to fight his fight. But I believe that if I had boxed him I'd have beaten him.
"I don't have the appetite to go through it all again. But I don't want to move too far away from boxing and I will possibly train fighters in the future."