Coolidge studied law at Hammond and Field, a law firm in Northampton, Massachusetts. He learned the tools of the legal trade from seasoned attorneys John C. Hammond and Henry P. Field. These fellow Amherst graduates took the young Coolidge under their wings.

Coolidge became a licensed attorney admitted to the Massachusetts bar in 1897 at the age of twenty-five. He opened his own law practice and worked effectively as an advocate for various business clients.