Tina Glover, of Kenmare Road, Knowle, pleaded guilty to assault at Bristol Magistrates' Court.
The court heard that Glover, 30, had been working as security at Panache nightclub, when the incident took place on Saturday, January 22.
The club, in All Saints Road, was closed last April and last month lost an appeal against the loss of its licence and has been closed down permanently.
Laura Opie, prosecuting, said: "The complainant had been in an argument with her boyfriend and had been taken to the coffee room, a quiet area of the nightclub.
"She was asked for her ID and we have CCTV of the defendant sitting on the victim's lap detaining her. She sat on her for 14 minutes although she was asked to get off."
Ms Opie said Glover admitted using unreasonable force and the claimant had been bruised on her leg and arms. She said that the complainant had "not given permission" for Glover to sit on her lap, had not been aggressive and had asked her to get off.
Nicola Hutchinson, defending, said Glover, who is now training to be a hairdresser, had attempted to take control of a situation in which the claimant was drunk and involved in an argument with her boyfriend.
She said: "Her intention was to calm her down, phone for a taxi, wait for the taxi and take the registration number down before logging the incident in the nightclub's book. Her concern was for the female."
She said that when Glover had taken her to the room the claimant began to shout, swear and grab her hair, though this was not captured on CCTV.
She said: "She sat on the claimant to calm her down and didn't realise the amount of time that had passed.
"She had never used this tactic before and there were a number of management in the coffee room at the time. The CCTV shows a scene of chaos. They did not tell her to get off."
Ms Opie said the claimant had reported finding it difficult to sleep at night thinking about the incident and worrying about repercussions.
She said: "She doesn't want to go into Bristol, even to shop, and is suffering from psoriasis due to stress."
Passing a sentence of a year's conditional discharge, the chairman of the magistrates said: "On the face of it this sounded like a very unpleasant assault and you have pleaded guilty at the earliest opportunity.
" I don't believe you acted with any malice but you used excessive force."
Glover was fined £135, including £50 compensation to the claimant.