The process begins Monday, when the old brick building that was the original shelter built in 1964 is being taken down.
Executive Director Suzan Prendergast said it served as the Bangor Humane Society for about 30 years and about 10,000 animals a year passed through its doors.
"The entrance to the building will be moved to the parking lot and the front of this building will be renovated. Make it more open. Make animals happier, healthier. More space for our pets so we can save more lives," she said.
The shelter will be closed on Monday and Tuesday due to the demolition.