Markedly elevated adverse mental health symptoms were widely observed early in the coronavirus disease 2019 (COVID-19) pandemic. Unlike the U.S., where cross-sectional data indicate anxiety and depression symptoms have remained elevated, such symptoms reportedly declined in the U.K., according to analysis of repeated measures from a largescale longitudinal study. However, nearly 40% of U.K. respondents (those who did not complete multiple follow-up surveys) were excluded from analysis, suggesting that survivorship bias might partially explain this discrepancy. We therefore assessed survivorship bias among U.S. respondents invited to complete multiple mental health surveys during the pandemic. Survivorship bias was assessed for (1) demographic differences in follow-up survey participation, (2) differences in adjusted initial adverse mental health symptom prevalences, and (3) differences in follow-up survey participation based on mental health experiences. Adjusting for demographics, individuals who completed only one or two out of four surveys had higher prevalences of anxiety and depression symptoms in April 2020 (e.g., one-survey versus four-survey, anxiety symptoms, adjusted prevalence ratio [aPR]: 1.30, 95% confidence interval [CI]: 1.08-1.55, P =0.0045; depression symptoms, aPR: 1.43, 95% CI: 1.17-1.75, P =0.00052). Moreover, individuals who experienced incident anxiety or depression symptoms had higher odds of not completing follow-up surveys (adjusted odds ratio [aOR]: 1.68, 95% CI: 1.49-2.48, P =0.0015, aOR: 1.56, 95% CI: 1.15-2.12, P =0.0046, respectively). Survivorship bias among longitudinal mental health survey respondents may lead to overly optimistic interpretations of mental health trends. Cross-sectional surveys may therefore provide more accurate estimates of population-level adverse mental health symptom prevalences. Assessment of survivorship bias among 4,039 mental health survey respondents revealed that individuals who participated in fewer surveys had higher prevalences of anxiety and depression symptoms at the first timepoint and worse mental health trajectories over time. Individuals who experienced incident anxiety or depression symptoms in May 2020 after not having experienced these symptoms in April 2020 had higher odds of not completing subsequent follow-up surveys compared with those who did not experience these symptoms (adjusted odds ratio [OR]: 1.68, 95% CI: 1.49-2.48, P =0.0015, aOR: 1.56, 95% CI: 1.15-2.12, P =0.0046, respectively). Restricting analytic samples to only respondents who provide repeated assessments in longitudinal survey studies may introduce survivorship bias, which could lead to overly optimistic interpretations of mental health trends over time.Cross-sectional or planned missing data designs may provide more accurate estimates of adverse mental health symptom prevalences than longitudinal surveys.