The phrase is thought to have originated in the United States in the 19th century, when sheriffs were elected officials responsible for law enforcement in their respective counties. When a new sheriff was elected, they would often bring in their own deputies and make changes to the way law enforcement was carried out. This could lead to significant changes in the local community, as the new sheriff might have a different approach to law enforcement or might prioritize different issues than the previous sheriff.
Over time, the phrase "a new sheriff in town" came to be used more broadly to refer to any new person in a position of authority who brings about changes or has a significant impact on their area. It is now commonly used in a variety of contexts, such as politics, business, and sports.