Lake Cassidy is a census-designated place (CDP) located in Snohomish County, Washington, United States. As of the 2020 census, Lake Cassidy had a population of 3,757.
The population of Lake Cassidy was 3,415 at the 2010 Census. 1,880 people are male. 1,535 are female.
Lake Cassidy is located at coordinates (48.066044, -122.082433). The elevation is 371 feet.
According to the United States Census Bureau, the CDP has a total area of 10.8 square miles (27.9 km<sup>2</sup>), of which, 10.6 square miles (27.4 km<sup>2</sup>) of it is land and 0.2 square miles (0.6 km<sup>2</sup>) of it (1.98%) is water.