Mute Swans (Cygnus olor) are a charismatic yet invasive species, initially introduced to urban areas on the East Coast for their aesthetic appeal. These territorial and aggressive birds threaten other waterfowl, native wildlife, and human recreationists. Native vegetation and water quality is also impacted by their overgrazing and uprooting of aquatic plants. As Mute Swans expand westward and populations become naturalized, they consistently prove to be pests requiring management or eradication. The California Department of Fish and Wildlife has classified the Mute Swan as an invasive species, yet little is known about the species distribution and ecology in California, and their spread remains unchecked in the state. In this study, we document the geographical expansion, locate known breeding populations, and characterize habitats preferred by Mute Swans in California using data from our research, coupled with iNaturalist and eBird citizen science data. Mute Swans were first documented using those platforms in California in Los Angeles in 1951. Today, this invasive species has spread to 47 of the state’s 58 counties (81%), with potential breeding activity documented in 22 counties (38%). These data will help inform future management practices by wildlife management agencies.