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

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 interim for one year following the retirement of Mary Dunbar, after many years as a science and math teacher at Forsyth. 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), thereby entering 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.

Dr. Rebecca Glenn led Forsyth through a period of expansion that encompassed every aspect of the school, with support from a highly 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, reaching 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. 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 a 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 campus facilities plan during this time. Campus expansion included the acquisition of two adjacent properties and an accompanying capital campaign 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.

Timothy Burns served as interim for one year following Mike Vachow's departure.

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 a new Eyas Program for ages 2–3, which served 13 children in its inaugural year. Total enrollment remained stable at 355–365 students.

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 Division Science Lab on the third floor of New House. The Forsyth Boulevard parking lots were renovated in 2021.

FRobert Butler joined Forsyth in the summer of 2021. Innovation & Design (I&D) was officially launched in 2022/23 as a new specialist subject area for students in Senior-Kindergarten through Grade 6. Enrollment reached 394 during the 2025/26 school year.