Hewitt came to the United States in 1902 as a private tutor to a prominent family in Tuxedo Park, New York.

After a decade in that position and at the suggestion of the Hoffman family, Hewitt began private classes for children in a townhouse on the Upper East Side.

By 1920, Hewitt had established a small kindergarten for boys and girls located at the Mannes Music School.

In 1986 a major renovation of the 75th Street Building was completed, adding classroom space and the John and Elizabeth Hobbs Performing Arts Center.

In 2001 the Hewitt community went online, and a revised course of study based on the curriculum mapping process was initiated.

In 2002, with a gift from the McKelvey Foundation, Hewitt purchased another townhouse to accommodate the Lower School, beginning in the fall of 2003.

In her five-year tenure, Lonergan led the expansion of the school; the townhouse to the west of the Gregory Building was purchased.