There are a number of different careers a person with such a degree could pursue. The careers would depend to some degree on what sort of a specialty a person pursued within criminology.
For example, a person specializing in scientific aspects of criminology could become a forensic scientist. A person with strong quantitative and statistical skills could find work trying to forecast crime levels in various places for governments or even for private companies that might be interested in such things. A person with managerial skills to go along with the criminological background could go into corrections at a managerial or planning level (as opposed to being a guard). A masters in criminology would help a police officer rise in that profession. A person who focused more on sociological issues could work in programs like "Weed and Seed" that are meant to help prevent crime in a given area of a city.
There are many careers open to a person with a masters in criminology, depending on their skills and interests.