Albert Dawson - who has also acted as the club's head groundsman - attacked 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.
Fiscal depute Eilidh Robertson described the subsequent assault to Dundee Sheriff Court. Ms Robertson added: "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."
The woman then raised a complaint with her line manager at the club - but said she did not want the police involved at that point.
Dawson was confronted by club bosses and admitted his behaviour - telling them: "I grabbed her from behind."
Dawson was only given a warning and was allowed to keep his job.
Ms 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 comment towards her that made her feel uncomfortable and it was at this point she decided to report the incident to police."
Dawson, 49, of Forfar, pled guilty on summary complaint to a charge of sexually assault. Sentencing was deferred until next month for reports.