The gardens are named after Thelma and Charles Mercer, who purchased 14.5 acres (59,000 m2) in the late 1940s for their home and garden. They preserved native trees such as dogwoods, rusty blackhaw viburnum, and hawthorns, and introduced camellias and exotic tree species such as ginkgo, bauhinia, philadelphus, camphor, and tung oil. The property was purchased by Harris County in 1974, with major expansions of 200 acres (0.81 km2) in 1984, and a further 38 acres (150,000 m2) in 1993.
Today's gardens include Azalea Berms, Bald Cypress Pond, Bamboo Garden (with more than 30 species of spreading and clump-forming bamboo), Daylily Garden, Endangered Species Garden, Fern Garden, Ginger Garden, Hawthorn Loop, Herb Garden, Hickory Bog, Iris Garden, Lily Pond, Maple Collection, Post Oak Uplands, Prehistoric Garden, Rock Garden, Salvia Collection, Shade Bog Garden and Boardwalk, Tropical Garden, Vine Garden, and a Woodland Pond and Bridge, as well as picnic tables and more than 3 miles (4.8 km) of walking paths.
360-degrees panoramic view of Mercer Arboretum and Botanical Gardens