The School opened in 1891 as Williams Memorial Institute (WMI), which remains the legal name of the school. It was privately endowed by a bequest of real estate and personal property from the estate of Harriet Peck Williams as a memorial to her son, Thomas W. Williams II. The School was created as a high school for “the promotion and advancement of female education" at a time when few saw it as a priority.
WMI served as the high school for girls of New London and several surrounding towns. With the opening of New London High School in 1951, the WMI program became college preparatory rather than comprehensive. In 1954, the School moved from Broad Street to its present location on the Connecticut College campus.
Other major changes for Williams were the addition of a middle school (grades 7 & 8) in 1955. In 1971 the School became coeducational. In 2014 the School expanded its middle school, adding a Sixth Grade class, and moved from an AP curriculum to a distinctive Advanced Courses curriculum.
The School is now an independent college preparatory day school with approximately 210 students from over 40 local communities in Connecticut, Rhode Island, and China.
The Williams School does not discriminate on the basis of race, color, religion, national origin, gender, gender identity, sexual orientation, age, marital status, veteran status, disability status, or any other characteristic protected by law.