Mary Dunbar recruited teachers who shared her philosophy and vision for an environment of creative learning that emphasized the importance of independence, confidence, and self-discipline. Forsyth grew quickly from 14 preschool children in 1961 to an enrollment of 270 students in Pre-Kindergarten through Grade 6 by 1970. In 1974, the Independent Schools Association of the Central States (ISACS) granted Forsyth School accreditation.

Fillmore House was acquired in 1965 and officially purchased in 1968. Dunbar House was acquired in 1977 for Grades 2–6.

Ellen Mutrux served as director for one year between Mary Dunbar and Barbara Skimming after working as a science and math teacher at Forsyth for many years. Together, Ellen and Mary developed Forsyth’s Summergrowth program.

Barbara Skimming focused on evaluating and improving the curriculum and facilities to best prepare students for secondary school. Admission testing also began. A Social Studies curriculum was developed, grammar and spelling were emphasized, and French and Latin were taught. Forsyth became part of the Education Confederation of St. Louis, the Missouri Association of Independent Schools, and the National Association of Independent Schools (NAIS) and entered the mainstream of the independent school community.

A two-story addition to Dunbar House was completed in 1982; additions to Fillmore House and playground improvements were completed in 1985–86. Also in 1982, the Main Office was relocated to Dunbar House, where it remains today.

Rebecca Glenn led Forsyth through a period of expansion that encompassed every aspect of the school, thanks to the support of a very entrepreneurial Board of Trustees and administration. During this era, the campus doubled in size; old facilities were improved, and new facilities were added. Forsyth invested substantially in curriculum development and alignment, as well as the technology for curriculum support. Finally, financial aid and fundraising were both instituted. Enrollment grew with the campus and reached nearly 400 students in 2006/07. Forsyth emerged as a highly respected, leading independent elementary school in St. Louis.

New House was purchased and renovated in 1989; Gerdine House was purchased and renovated in 1995; Last House was bought and renovated in 1996; and the Rand Center for Performing Arts & Athletics was built in 1997. (The Dunbar House addition was torn down to make room for the new Rand Center.) A four-classroom addition to Last House was completed, and the Sport Court was built in 1998–99. Falcon Field and the Williams Walled Garden were built in 2004. The Adventure Center with high ropes course, climbing wall, and high beam (built in 1998) was constructed in the Rand Center in 2006. The Rebecca Glenn Playgrounds opened in 2007.

Michael Vachow focused on refining the organizational structure and systems appropriate to a mature educational institution. Forsyth completed two strategic plans and developed a facilities master plan during this time. Campus expansion included the acquisition of two adjacent pieces of property, facilities planning, and an accompanying capital campaign to raise funds for campus improvements and endowments.

Forsyth purchased a ½-acre lot from St. Louis Hillel at Washington University in 2011 and transformed the land into the Big Backyard in 2013. Next House was purchased in 2012; Last House and Next House were renovated in 2013. Fillmore House was fully renovated in 2015.

Following Mike Vachow's departure, the Forsyth School Board of Trustees announced the appointment of Timothy Burns as the Interim Head of School for the 2017/18 school year.

Daniel Hildebrand led Forsyth through the global COVID-19 pandemic of 2020/21, reaccreditation with the Independent Schools Association of the Central States (ISACS), and the adoption of a new mission, vision, and values through a strategic planning process. Also in 2020/21, Forsyth developed and launched its new Eyas Program for ages 2–3, which served 13 children in its inaugural year. Total enrollment remained stable at 355–365 students during this time. Finally, Forsyth initiated a renewed emphasis on diversity, equity, and inclusion.

The stand-alone Science Center adjacent to Fillmore House was transformed into the Falcon’s Nest in 2020 for the Eyas Program. The Science Center was relocated to a newly renovated Upper School Science Lab on the third floor of New House, across the fire escape from the Lower School Science Lab on the third floor of Fillmore House. The Forsyth Boulevard parking lots were renovated in 2021.

Forsyth welcomed Robert Butler, Jr., as the sixth permanent Head of School in the summer of 2021 following the retirement of Daniel Hildebrand. Innovation & Design was officially launched as a new specialist subject area in 2022/23 for students in Senior-Kindergarten through Grade 6. That same year, Forsyth established a new administrative leadership position for community and culture. Forsyth added a part-time teaching position dedicated to early childhood music for the 2023/24 school year.