Background: : The impact of the coronavirus disease 2019 (COVID-19) on the mental health of the US population is unclear. This study drew on two nationally representative samples to compare the prevalence rate of anxiety in the U.S. before and during the 2020 COVID-19 pandemic. Methods: : The Generalized Anxiety Disorder-2 (GAD-2) screening tool was used to detect the proportion of US adults screening positive for high levels of anxiety symptoms. Anxiety symptoms was assessed in 2019 using the National Health Interview Survey (NHIS; N = 30,915) and during the pandemic using biweekly surveys collected as part of the Understanding America Study (UAS; N=8,022 Obs.=121,768) between March and December 2020. Results: : The proportion of participants with high levels of anxiety symptoms increased significantly from 8.1% (95% CI[7.7, 8.5]) in 2019 to 21.4% (95% CI[19.9, 22.9]) at the beginning of April, 2020. The prevalence then declined to 11.4% (95% CI[10.3, 12.5]) in May and remained 3% above 2019 levels until December 2020. This pattern of increasing anxiety between 2019 and April 2020 followed by a rapid decrease in anxiety was identified across all demographic characteristics examined. Limitations: : The NHIS and UAS samples differ in their sampling and mode of administration which may bias comparisons between samples. Conclusions: : Anxiety symptoms increased markedly during the onset of the COVID-19 pandemic and reduced quickly as stay-at-home orders were lifted. These findings highlight the importance of providing mental health supports during future lockdowns and suggest that resilience in mental health may have been a key population level response to the demands of the pandemic.