In January of 1926, seven parents came together to develop a new school, united by a shared belief in education, character, and community. Among them was Joseph P. Kennedy, whose sons Joseph Kennedy, Jr., and John F. Kennedy, would attend the school.

In the fall, Dexter School for Boys opens, occupying a site in Brookline during the headships of Miss Myra Fiske and the Reverend Francis Caswell.

Under Headmaster William F. Phinney, the School purchases the Newton Street property and begins construction of the current campus. Classrooms, athletic facilities, and art studios follow.  

Southfield School is established to offer girls the same outstanding education that Dexter boys have long enjoyed. Southfield opens its doors to 81 girls in PreK–Class 4, far exceeding the planned enrollment. Within four years, Southfield numbers 236 girls through Class 8.

The Upper School opens, providing families with a continuous PreK through Class 12 experience. The School expands the faculty, facilities, and program to serve the growing student population.  

Mr. and Mrs. Phinney retire, having given a combined 93 years of service to the School. We remember their legacy with deep gratitude following Mr. and Mrs. Phinney's passing in 2026 and 2023, respectively.

Longtime teacher, administrator, and coach Todd Vincent becomes the fourth Head of School.

In Summer 2013, Dexter Southfield unveils its re-branded school name to reflect the shared community identity. 

The Kraft Fieldhouse & Fusco Arena, including 100,000 square feet of indoor space, opens to the Dexter Southfield community. The space includes courts for basketball, volleyball, and tennis; a renovated hockey arena, including a multipurpose room and seating; dedicated areas for strength and conditioning and athletic training; and several locker rooms.

The Building a Better Tomorrow project adds 23,000 square feet of academic spaces across divisions, including the Gui Building for Classes 3–5, and enables strategic growth in the Upper School.

Dexter Southfield announces three new program enhancements for the following school year: The Dexter Southfield Squash Center, The Garage for middle school innovation, and the New Venture Competition and the entrepreneurship program in the Upper School. 

Embarked on a two-year Academic Excellence and Curriculum Alignment Study from PreK through Class 12.

The Building a Better Tomorrow: Phase II initiative adds three new facilities, 20 new faculty positions, and increased academic, athletics, and extracurricular offerings.

On September 2, 2025, the School celebrated its inaugural Academic Convocation and installed Dr. Peter Folan as the fifth Head of School.

On January 8, 2026, the student body gathered to celebrate the hundredth anniversary of Founders' Day.