Nationally recognized for its distinctively Canadian style, Runnymede Branch was designed by John M. Lyle, one of this country's most distinguished 20th-century architects. In the 1920s, a surging sense of national pride inspired Lyle to create a uniquely Canadian architecture that blended European styles with Canadian themes and ornamentation. Runnymede Branch was an early attempt at such a design.

The building is constructed of variegated red and yellow Credit Valley stone, and combines Georgian, French, and early Quebec styles, the latter in its steeply pitched, hipped roof. Lyle used Canadian aboriginal motifs for much of the decoration, including totem poles at the main entrance and arrowheads in the iron railing above. Carvings of native plants and animals also embellish the building. In 1989, the Runnymede Branch was featured on the first in a series of postage stamps celebrating Canadian architecture. The building was most recently restored and enlarged in 2005.