Dorchester is a planning district comprising more than 6 square miles in the city of Boston, Massachusetts, United States. Originally, Dorchester was a separate town, founded by Puritans who emigrated in 1630 from Dorchester, Dorset, England, to the Massachusetts Bay Colony. This dissolved municipality, Boston's largest district by far, is often divideā¦Dorchester is a planning district comprising more than 6 square miles in the city of Boston, Massachusetts, United States. Originally, Dorchester was a separate town, founded by Puritans who emigrated in 1630 from Dorchester, Dorset, England, to the Massachusetts Bay Colony. This dissolved municipality, Boston's largest district by far, is often divided by city planners in order to create two planning areas roughly equivalent in size and population to other Boston districts.