Williams Memorial Institute was opened in 1891 through a trust fund bequeathed by Mrs. Harriet Peck Williams to establish a free girls' school in memory of her son, merchant Thomas W. Williams II who died in 1855. The trust empowered trustees to build a school when $150,000 had accrued in the account. In 1881, trustees announced that $60,000 would be spent to erect the building. However, by the time its doors opened in 1891, the cost had grown to $85,000.

The building housed the high school for New London girls until 1951, when the City opened a coeducational high school. The Institute remained open for three more years as a college preparatory school. In 1954, the school moved to its present location on the Connecticut College campus, where it operates as a coeducational college preparatory day school for students in grades 7-12.

In 1972, the State of Connecticut signed a 25-year lease and the Judicial Branch soon opened a Courthouse in the building once called the Williams Memorial Institute. In 1997, the State purchased the building for $200,000 and began renovations.