Early 20th-century Toronto suffered water shortages and unclean drinking water, causing public health advocates to campaign for a modern water purification system.

Construction for a water treatment plant began in 1932 and it became operational in 1941. Unlike most modern engineering structures, the building was also intended to make an architectural statement. Fashioned in the Art Deco style, the cathedral-like structure remains one of Toronto's most admired buildings. The opulent interiors have marble entryways and vast halls filled with pools of water and filtration equipment. The plant is nicknamed "The Palace of Purification".

In 1992, the R. C. Harris Water Treatment Plant was named a national historic civil engineering site by the Canadian Society for Civil Engineering. In 1998 it was designated under the Ontario Heritage Act.

The plant is fully functional, providing about 30% of Toronto's water supply.