Background: The lasting effects of the coronavirus disease 2019 (COVID-19) pandemic are likely to be significant, particularly in regard to mental health and wellbeing. The current study aimed to track worry and rumination (perseverative cognition) levels during the COVID-19 pandemic and to investigate whether periods with higher COVID-related worry and rumination were associated with more negative mental health outcomes in the UK population. Methods: Quota survey design and a sampling frame that permitted recruitment of a national sample was employed. Findings for waves 1 (31 March-9 April 2020), 2 (10 April-27 April 2020), 3 (28 April-11 May 2020), 4 (27 May-15 June 2020), 5 (17 July-7 August 2020) and 6 (1 October-4 November 2020) are reported here (N=1943 adults). At each wave COVID-related worry and rumination were assessed together with depression, anxiety and mental wellbeing.Results: Worry and rumination levels were highest at the beginning of the first UK lockdown and declined across the six waves but increased when the UK returned to lockdown. Worry levels were significantly higher than rumination levels throughout. COVID-related worry and rumination were significantly associated with higher symptoms of depression, anxiety and lower wellbeing. The adverse effects of COVID-related worry on depression and anxiety levels were most marked in individuals living with a pre-existing mental health condition. Conclusions: Psychological interventions should include components that specifically target COVID-related worry and rumination. Individuals with pre-existing mental health conditions should be prioritised as we continue to emerge from the current pandemic and in any future public health crises.