Background: There is increasing evidence of the negative impact of the COVID-19 pandemic and resultant shutdowns on mental health. This issue may be of particular concern to those affected by intimate partner violence (IPV) and sexual violence. Methods: We conducted a cross-sectional analysis using data from a California state-representative online survey conducted in the two weeks following the state stay-at-home order, enacted March 19, 2020 (unweighted N=2081). We conducted a series of multivariate multinomial logistic regressions to assess the associations between a) days since stay-at-home order and b) gender-based violence exposure with anxiety and/or depression symptoms, accounting for demographics, and secondarily demographics and social support. Findings: Nearly one in five (17·9%) respondents reported moderate or severe mental health symptoms in the past two weeks; 15·7% had a history of IPV and 10·1% had a history of sexual violence. In models adjusting for gender, gender-based violence history, and other demographics, days since shutdown was significantly associated with greater mental health symptom severity, as were IPV and sexual violence. When social support was additional controlled for, however, effects of days from shutdown were lost and effects related to violence were slightly attenuated. Interpretation: The odds of poor mental health symptoms increased daily under COVID-19 shutdown conditions in this study, and for those with a history of IPV. These findings were respectively lost and attenuated after adjusting for social support, suggesting the value of social support programs to address mental health concerns under the pandemic. Funding: Blue Shield Foundation of California Grant RP-1907-137. Bill and Melinda Gates Foundation OPP1179208.Declaration of Interests: The authors have no conflicts of interest to declare.Ethics Approval Statement: All research procedures were approved by both NORC/University of Chicago and the University of California San Diego Institutional Review Board (Project #191904XX).