Frank Cairney, 87, preyed on the teenage boys between July 1978 and June 1989.
Cairney struck in the Celtic Park shower room and dressing room of the club's Barrowfield training ground.
The convicted sex offender also carried out his abuse on a team trip to Aviemore, Scottish Highlands, and in his car on numerous occasions.
Cairney, who was installed in his role by club legend Jock Stein, received a three-year sentence in 2019 for abusing seven boys over 20 years.
Further allegations emerged after three further victims contacted police in 2018 and 2019.
Cairney, of Uddingston, South Lanarkshire, was due to stand trial charged with three indecent assaults.
Glasgow Sheriff Court heard earlier this week that Cairney was unfit to stand trial and an examination of facts hearing was set in its place.
Earlier hearings were told Cairney suffered from "health problems" including dementia.
His attendance was also excused from the examination of facts.
The court heard from a 52-year-old man who stated he suffered abuse at the hands of Cairney from early 1986.
The man who signed for Celtic stated he was directed by the club to play for Cairney's Celtic Boys Club under-16 team.
He stated that Cairney was a "big imposing man" and "a bit of a bully" who was verbally aggressive towards his team.
The witness claimed he was "homesick" and "upset" following a match he played at Celtic Park and took refuge in the shower room.
He stated that he was joined alone by Cairney who followed him in.
The man said: "He put his arms around me to hug me in a consoling way I thought at the time.
"He put his hand on my lower back and his other hand was at the top of the inside of my leg.
"He then moved his hand and touch my private parts and lower stomach."
The man stated that he was still wearing his football kit at the time.
Cairney went on to attack the boy again while they were alone in the Barrowfield changing rooms.
The man said: "He hugged me once again and he would have put his hand on the inside of my leg similar to the first occasion.
"Then he rubbed my neck and moved his hand inside my football gear and touched my private parts - it was longer that time."
Cairney groped the man on a later occasion while the pair were alone in a "secluded" tea room at Celtic Park.
The witness said: "He proceeded to hug me once again and he again put his hand on my back.
"He put a hand against my leg on the outside of my clothes and touched my private parts once again."
The man stated that he later reported Cairney to one of Celtic's Irish scouts in 1988 who then forwarded him on to the club's chief scout but nothing came of it.
A 50-year-old victim told the court that Cairney performed a sex act on him while in a hotel room on a team trip to Aviemore between 1988 and 1989.
He stated: "I think he asked me to go to his room before or after dinner under the pretence of what happened during the game.
"I remember talking about football in general.
"I don't know if I was leaving, I stood up and he stood up and he touched my clothing and put his hand down the front of my clothing."
The man stated that he was also touched on the groin on numerous occasions when Cairney would drive him home from football training at Barrowfield.
A 59-year-old victim stated he was touched on the thigh on two occasions by Cairney between 1978 and 1979.
He recalled the first time was in Cairney's car when he gave him and two teammates a lift to Paisley, Renfrewshire, for a match against St Mirren.
The victim said: "He had his back to the passenger door and he put his hand on my thigh."
He added that his teammates looked out of the window when it was happening.
The man stated he "freaked out" and "just froze".
The witness recalled an aggressive Cairney also touched him on his thigh "three or four" times in the Barrowfield changing rooms.
The man said: "He would slap someone, he would plant himself between the players and he would put his on my thigh again.
"The rest of the lads wouldn't say something - they would look away."
Prosecutor Mark Allan asked what impact the incidents had on the victim.
He replied: "My mental health has been all over the place ... it never goes away - 44 years of it."
The court heard that some of the victims did not come forward due to Cairney's influence on the team and the influence it would have on their careers.
Cairney's recorded police interview following his arrest in August 2019 was played to the court.
He told officers that he was installed as manager of Celtic Boys Club by legendary first-team boss Jock Stein in 1970.
Cairney said: "[Stein] says you are not going to Hamilton Accies, you will take the Celtic Boys Club under 16s.
"I will put s-forms [schoolboy players] in there and you are responsible."
Cairney later stated: "It was called Celtic Boys Club but it was Celtic under 16s."
He added that he was assisted by Lisbon Lion captain Billy McNeill "for a few months".
Cairney told the officers that Hoops legend Tommy Burns, Charlie Nicholas and Derek Whyte took training sessions while doing SFA coaching badges.
Cairney gave a largely "no comment" response to sexual abuse allegations before stating: "It never happened."
He was cleared mid-trial of physically assaulting one of the abuse victims after the charge was pulled.
Giving his verdict, Sheriff Tom Hughes said: "I took the view that the crimes charged were so closely linked by the character, the circumstances of their commission and in time as to bind them together as part of a single course of criminal conduct systematically pursued by Cairney.
"On the balance of probabilities, there are no grounds for acquitting Cairney and I make findings to that effect.
"The victims gave evidence on what must have been very difficult circumstances for them.
"They gave their evidence with great dignity and I hope they can put the circumstances of these offences behind them and start to move on with their lives.
"They are an example to others that it is possible to bring these horrendous crimes to the attention of the court no matter how long ago they occurred.
"These matters are always taken seriously by the courts and they should have the consolation of knowing that they came to court and were believed. I hope this is some consolation to them."
Mr Allan did not move for sentence after the verdict and asked that the case be "deserted pro loco et tempore" due to the legal provisions of Cairney being unfit for trial.
He added: "This is on the basis of the medical reports before the court."
Sheriff Hughes granted the motion which was not opposed by Mr Crosbie.