The Douglas-fir tussock moth (DFTM), Orgyia pseudotsugata McDunnough (Lepidoptera:Erebidae:Lymantriinae), is a native western North American defoliator of true fir, Abies spp. Mill., and Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. We investigated the population genetics and impact associated with the first recorded outbreak of DFTM in southern California (USA), and report the first collection of DFTM in Baja California Norte, Mexico. This latter population is similar genetically to populations from Washington, USA and British Columbia, Canada. We assessed forest stand characteristics, levels of defoliation, and mortality of white fir, Abies concolor Lindl., associated with the DFTM outbreak in the Transverse Mountain Ranges of southern California. We compared these data to those from southern California non-outbreak stands of A. concolor, and from virgin stands with an A. concolor component in the Sierra San Pedro Martir National Park (Mexico). Total stand density (ha−1) was significantly higher (22%) in non-outbreak stands than in outbreak stands. However, outbreak stands had significantly higher mortality of A. concolor than non-outbreak stands [whether expressed as density (70%) or basal area (m2 ha−1) (32%)]. Total stand and A. concolor density and basal area for living and dead trees were significantly lower in the Sierra San Pedro Martir National Park than in southern California. Dead A. concolor comprised >95% of all tree mortality in both outbreak and non-outbreak areas in southern California, which corresponded to a mean 20% basal area loss of A. concolor associated with DFTM feeding injury within the outbreak area. The mean level of defoliation of A. concolor by DFTM was 39%, and 62% of all dead A. concolor were associated with DFTM defoliation. In stands with high levels of defoliation, larval feeding and tree mortality were also noted in Jeffrey pine, Pinus jeffreyi Grev. & Balf. The amount of dead A. concolor basal area associated with the fir engraver, Scolytus ventralis LeConte (Coleoptera: Scolytidae), in non-outbreak stands was 96% greater than in outbreak stands. Using the U.S.D.A. National Insect and Disease Risk Map software, a total of 13,534 ha were predicted to be at risk to basal area loss from future DFTM outbreaks on national forest lands in southern California. Changes in forest management practices and fire suppression policies likely led to an increase in the density and continuity of DFTM’s preferred host in southern California and to a southward shift in the historic range of DFTM outbreaks.