Albert Dawson - who also acted as the club’s head groundsman - launched the attack on the woman in the East Stand at the Premiership club’s ground in 2011.
Dawson walked up behind her as she carried out her job near a catering kiosk and said: “What are you doing?”
He then stood behind her and put his arms around her, pulling her close to him.
A sheriff branded his behaviour “totally unacceptable” - and told him he could still face jail if he doesn’t comply with a community payback order.
Fiscal depute Eilidh Robertson told Dundee Sheriff Court: “He then touched her breasts over her clothing, put his hands between her legs and cupping her genitals over her clothing for a few seconds.
“He thereafter walked away as she became extremely shocked and upset.
“She ran out of the stand and found a groundsman who was nearby and told him what was happening.
“The groundsman could see she was upset.
“She subsequently went home, still upset and crying, and told her partner about the incident.
“She said ‘that dirty b*****d had his hand between my legs’.”
The woman then raised a complaint with her line manager at the club - but said she didn’t want the police involved at that point.
Dawson was then confronted by club bosses and admitted his behaviour - telling them: “I grabbed her from behind.”
But despite admitting the attack Dawson was only given a warning and was allowed to keep his job.
He was eventually sacked last month after he admitted the attack.
Miss Robertson added: “The accused did not speak to the complainer for some time, however at the start of last football season he made a further coment towards her that made her feel uncomfortable and it was at this point she decided to report the incident to police.
“She was hysterically upset while disclosing the matter to them.
“The following week she left her job and went on sick leave.
“The accused was interviewed in relation to this matter by police and said ‘I held my hands up to that - I had been mucking about with the lass and I went too far’.
“He admitted he had cupped her genitals and couldn’t provide any defence for his actions.”
Dawson, 49, of Victoria Street, Forfar, pleaded guilty to sexually assaulting the woman at Tannadice Park on one occasion between February 1 2011 and April 30 2011.
Bob Bruce, defending, said: “This was done as an act of horseplay that went too far.
“His position is he would often goof around with work colleagues and would go up behind colleagues - male and female - and tickle them.
“He accepts this started out as a joke but he went too far and that’s not acceptable.
“He’s lost his job having worker there for a long time and the loss of his job has caused him great anguish.
“A custodial sentence would simply be devastating.”
Sheriff Tom Hughes placed Dawson on a three year community payback order with supervision and placed him on the sex offender’s register for three years.
He said: “It is clear to everyone that your behaviour was totally unacceptable.
“The position is that you placed the lady concerned in this attack in a very distressing situation.
“She is traumatised by this to this very day.
“If you breach the order it will come back to me - so if you don’t comply the other option will be a custodial sentence.”
Dawson made no comment as he left court today.
Dawson’s victim earlier said she was relieved that he had come clean in court.
She said: “I’m over the moon - it’s not a case of his word against mine any more.
“Relieved doesn’t cover it and it’s caught me off guard.
“Everybody said I was making it up and now it has been proven that I was telling the truth.
“These past two years have been a nightmare for me.
“I’m prone to panic attacks now and for the past month they have been getting worse.
“I regret not reporting it earlier. Now I can just get on with the rest of my life.”
Dawson looked after the pitch at Tannadice from 2000 until last month.
He became the first person to win the Scottish Groundsman of the Year ward, given out by the Institute of Groundsmanship.
He was given the award for the SPL as well as the overall prize for best in the country.