Westley is a census-designated place (CDP) in Stanislaus County, California, United States. The population was 603 at the 2010 census, down from 747 at the 2000 census. It is part of the Modesto Metropolitan Statistical Area. Westley lies at the base of the Diablo Range and in the vicinity of Ingram Creek. Somewhat nearby is the San Joaquin Fault. East of the San Joaquin Fault there is a flow pattern of alluvium reported as a mud flow. This flow pattern was deposited in the early Holocene period or the late Pleistocene.