When Patty is the only child of Penny and Allen Halliwell. At Patty's wiccaning (a ceremony similar to a baptism, but for witches) the Necromancer (the ghost of a demon who feeds on the lifeforce of magical beings) attacked trying to take the magic of the Halliwell line. But Penny banished the Necromancer which lasted about fifty years.
Patty was a very smart little girl, and when she was nine she started to write her own magic spells. And some of the spells were even to counter her mothers spell that were cast.
Marriage and the Girls
Patty met, fell in love with, and married a mortal man named Victor Bennett, although Patty's mother disapproved of her marrying a mortal. Patty's mother actually hated the thought of her daughter marrying a mortal so much that she refused to allow her daughter to take Victor's last name. With Victor she gave birth to three daughters, Prue (when she was 20), Piper (when she was 23), and Phoebe (when she was 25). Patty didn't tell Victor that she was a witch until after the marriage. Victor tried to keep and open mind when it came to Patty being a witch, although Victor and Penny constantly got into fights over how Prue, Piper, and Phoebe should be raised. Patty and Penny wanted the girls to be raised as witches to embrace their destiny, but Victor wanted his daughters to be raised as mortals to protect them from demons and other dark creatures. Eventually Victor and Patty got a divorce and Victor left San Francisco.
After Victor left Patty didn't have any money coming in so she became a waitress. Soon after this she was tricked by the warlock Nicholas into blessing a ring, giving Nicholas immunity from the Charmed Ones powers. To protect the girls from Nicholas Patty and Penny binded the girls powers to protect them.
Soon after the incident with Nicholas, Patty fell in love with her whitelighter (guardian angel for witches) Sam and conceived Paige. Because it was forbidden for witches and whitelighters to be together Patty gives Paige up because she fears that Prue Piper and Phoebe would be denied their powers if anyone found out about Paige. So Sam and Patty took Paige to a local church and left her with a nun named Sister Agnes.
Death and Afterlife
Sometime after Patty gave birth to Paige she got into a fight with the Water Demon at Prue, and Pipers summer camp. The demon killed Patty by drowning her, because Patty was distracted by Sam who tried to stop her from fighting the demon.
Just as Penny helps the Charmed Ones from the afterlife as a ghost Patty does as well, although not as much as Penny does.
Patty had the magical ability to freeze time, the same power her daughter Piper had. She rarely used her power in the episode that she appeared, so we don't know how powerful she was. As a witch, she could also cast spells and create potions.