Amidst the ongoing COVID-19 pandemic, people are facing heightened uncertainty about the future and increasing rates of psychological distress. Intolerance of uncertainty (IU) and perceived COVID-19 threat may be contributing to mental health problems. This study investigated changes in mental health problems prior to and during the first two pandemic waves in the U.S., and the extent to which IU and perceived COVID-19 threat predicted these problems. MTurk participants (n=192; 50% women) were recruited from a pre-pandemic study in December 2019/January 2020 for a follow-up study on COVID-19 experiences, across five timepoints between April and August 2020. IU, perceived COVID-19 threat, and mental health problems (i.e., worry, COVID-19 fear, and trauma symptoms) were assessed. On average, mental health problems were not elevated, relative to pre-pandemic levels, and remained stable across time. Heightened IU and perceived COVID-19 threat were associated with more mental health problems. Surprisingly, objective measures of COVID-19 threat (e.g., state case rates) showed no associations with IU, and were slightly negatively correlated with psychological distress and perceived threat. Pre-existing mental health symptoms, IU and perceived COVID-19 threat may foster vulnerability to mental health problems during the pandemic, more so than objective threat levels.