Mute swans (Cygnus olor) are an invasive species known to impact aquatic vegetation and native wildlife. Their recent expansion in California prompted the passage of Assembly Bill 764 in October 2025 which permits the take and possession of mute swans. To better understand this emerging population, we quantified the geographical and numerical expansion of mute swans in California using records from three major sources: eBird, Audubon’s Christmas Bird Count(CBC), and the California Department of Fish and Wildlife’s(CDFW) Waterfowl Breeding Population Survey. eBird data indicate initial introductions in urban centers of Los Angeles and San Francisco, followed by recent, rapid expansion into the San Francisco Bay–Delta and Central Valley. The proportion of occupied 1-km grid cells doubled between 2014 and 2024 and increased twentyfold since 2004. CBC data revealed similar patterns, with 11% of counts reporting mute swans in 2024 compared to 3% in 2004. Numerically, CDFW and CBC data both indicate exponential population growth, with estimated annual growth rates of 17% (DT = 4.4 years, 2000–2024) and 28.9% (DT = 2.7 years, 2007–2025), respectively. This rapid expansion mirrors populations in the eastern United States and may pose increasing risks to native waterfowl and wetland ecosystems.