Background: We estimated socioeconomic factors associated with food insecurity during the first year of the Covid pandemic in the UK and explored potential mechanisms explaining these associations. Methods: Data were from the April, July, and September 2020 waves of the UK Understanding Society Covid Survey. Food insecurity was measured as ‘not having access to healthy and nutritious food’ and ‘reporting being hungry but not eating’. Logistic regression estimated the relationship between socioeconomic factors and food insecurity. A decomposition approach explored if financial vulnerability and having Covid-19 explained associations between socioeconomics factors and food insecurity. Results: Single parents and young people aged 16–30 years had a higher odds of reporting both measures of food insecurity. Financial insecurity explained 5% to 25% of the likelihood of reporting being food insecure for young people and single parents depending on the food insecurity measure used. Experiencing Covid-19 symptoms explained less than 5% of the likelihood of being food insecure for single parents but approximately 30% of not having access to healthy and nutritious food for young people. Conclusion: Policies providing additional financial support may help to reduce the impact of Covid-19 on food insecurity in the UK.