Emergency services were called to the derelict Pagefield building to help someone who was injured while on the roof.
Police officers, paramedics and firefighters all went to the site on Sunday afternoon.
An ambulance service spokesman confirmed a female patient had a leg injury after being on the roof.
She was seen by staff from a rapid response vehicle and did not need hospital treatment.
The incident came after 30 teenagers were seen at the Pagefield building on Sunday.
Families at Mesnes Park were shocked to see youngsters, aged 13 to 16, running up and down the stairs and even sitting on the roof.
A mum, who was at the park with her five-year-old son, contacted the Post to raise her concerns.
She said: “I was absolutely gobsmacked. Words fail me. Everyone was standing looking at them, shouting for them to come off.
“In tranches they came away and went back. Some of them were dangling their legs over the edge. God forbid if any of them had fallen.”
She added: “It was so distressing to see. It beggars belief why you would put yourself in that kind of position.”
The privately-owned Pagefield building previously housed a mill and college.
It has become a hotspot for arson attacks and anti-social behaviour, with emergency services called there regularly.
Wigan Council demolished a derelict building on the site last year and fencing was erected by the owners.
Wigan Central councillor George Davies urged the owners to keep the gates locked and demolish outer buildings.
He said: “The summer months are coming and there are going to be kids all over the place. We have to make sure that the building is secured 24 hours a day.”
Karl Battersby, the council’s director for economy and environment,said: “For some time now we have been liaising with the owners of the Pagefield building over safety concerns. The building is privately owned and therefore the responsibility for securing the site adequately rests with the owners.
“We are extremely concerned about the safety of any person entering the site and the buildings. We would advise people to stay clear of the site and for parents to warn their children of the dangers of exploring the site. It should be remembered that this is private land and anyone entering the site is trespassing.
“We are aware of the incident that has occurred over the weekend and we are currently working with the owners, GMP and also the fire service to implement further security measures and secure the site.”