In 1923, New York financier Alfred Barmore Maclay (1871–1944), the son of Robert Maclay, and his wife, Louise Fleischman, purchased the 1,935 acres (783 ha) Lac-Cal quail-hunting plantation and adjoining land, creating a 3,760 acres (1,520 ha)-estate he called Killearn, after his ancestral village and birthplace of his great-grandfather in Scotland.

His wife continued their development, opened them to the public in 1946, and in 1953 donated some 307 acres (1.24 km2) of their estate, including the gardens, to a predecessor of the Florida Department of Environmental Protection.

In 1965 the gardens were renamed in Maclay's honor, to avoid confusion with the new adjacent development called Killearn Estates.

Trees include bald cypress, black gum, cyrilla, dogwood, hickory, holly, Japanese maple, oak, plum, redbud, Liquidambar, and Torreya taxifolia.

Other plantings include Ardisia, Aucuba, Zamia integrifolia, Rhododendron chapmanii, Gardenia, ginger, jasmine, Oriental magnolia, mountain laurel (Kalmia latifolia), Nandina, palmetto, sago palm, Selaginella, Wisteria, and Yucca filamentosa.