[3][4] She had previously taught at her mother's school in suburban Winchester, and began sending her own students to colleges in 1893;[5] enrollment reached 99 girls by 1900 and 225 by 1910.

[4] According to the school website, Winsor "wanted to prepare women to be self-supporting, and hoped they would be competent, responsible, and generous-minded.

[4] Designed by prominent Boston architect R. Clipston Sturgis, the campus included a library, science laboratories, art studios, and athletic facilities.

[4][8] The well-equipped campus reflected Winsor's strong base of support among the Boston elite.

[9] When the school formally incorporated in 1907, Harvard president Charles Eliot and Radcliffe dean Agnes Irwin were on its first board of trustees.

"[11] Mary Winsor also established the present structure of eight classes from grades 5–12, in addition to a ninth, post-graduate year (now discontinued).

Winsor has offered a scholarship program since it was incorporated in 1907, although as late as 1970, only "about ten" students received financial aid.

[23] In 2006, Boston magazine said that Winsor's college matriculation record was "unmatched" in Massachusetts, with over one-third of alumnae going on to Ivy League schools.

[28] The school announced another fundraising campaign in 2024, which aims to raise $100 million to support faculty salaries and student financial aid.

[36] The school offers 13 Advanced Placement courses, mostly in math, science, and world languages.

[40] In the Upper School, Winsor offers electives in drama, dance, visual art, and music.

[44] Winsor occupies a seven-acre campus in a fashionable neighborhood of Boston,[27] one of the most expensive real estate markets in the United States.

[45] In 2014, the Boston Globe estimated that Winsor's 4.4 acres of athletic fields were worth as much as $90 million.