Hannah Marriott, 16, found the white reptile curled up on a bath mat in her Rochdale home.
At first, the teen thought the snake was a joke, but when it moved she ran downstairs to tell her mum, Karen, who phoned the police because she couldn't get hold of the RSPCA.
Karen, 40, told the Manchester Evening News: "Hannah was calm at first. She can't even deal with spiders, but I don't think she thought it was real.
"She'd been looking at it for a few minutes until it moved. She thought I was having a joke.
"I felt like a wally calling 999 but I didn't know what to do. I'm frightened because I don't know how it got in.
"It was like something out of a film. I didn't know whether to laugh or cry. Everybody needed a wee, but we couldn't go."
Detective Constable Craig Wallace overheard the control room call and offered to help because he has experience keeping snakes.
He raced to the family's home and managed to capture the snake in a pillow slip.
It was taken back to the station before being transferred into the care of the RSPCA.
Det Con Wallace believes it may be a missing or escaped family pet which slithered into the house during the recent warm weather.
He said: "I keep snakes so I know a lot about them. It was a northern pine snake. They can give a nasty bite, but they're not venomous."