Abstract Objectives Excess mortality captures the total effect of the COVID-19 pandemic on mortality and is not affected by mis-specification of cause of death. We aimed to describe how health and demographic factors have been associated with excess mortality during the pandemic. Design Time-series analysis. Setting UK primary care data from practices contributing to the Clinical Practice Research Datalink on July 31st 2020. Participants We constructed a time-series dataset including 9,635,613 adults (≥40 years old) who were actively registered at the general practice during the study period. Main outcome measures We extracted weekly numbers of deaths between March 2015 and July 2020, stratified by individual-level factors. Excess mortality during wave 1 of the UK pandemic (5th March to 27th May 2020) compared to pre-pandemic was estimated using seasonally adjusted negative binomial regression models. Relative rates of death for a range of factors were estimated before and during wave 1 by including interaction terms. Results All-cause mortality increased by 43% (95% CI 40%-47%) during wave 1 compared with pre-pandemic. Changes to the relative rate of death associated with most socio-demographic and clinical characteristics were small during wave 1 compared with pre-pandemic. However, the mortality rate associated with dementia markedly increased (RR for dementia vs no dementia pre-pandemic: 3.5, 95% CI 3.4-3.5; RR during wave 1: 5.1, 4.87-5.28); a similar pattern was seen for learning disabilities (RR pre-pandemic: 3.6, 3.4-3.5; during wave 1: 4.8, 4.4-5.3), for Black or South Asian ethnicity compared to white, and for London compared to other regions. Conclusions The first UK COVID-19 wave appeared to amplify baseline mortality risk by a relatively constant factor for most population subgroups. However disproportionate increases in mortality were seen for those with dementia, learning disabilities, non-white ethnicity, or living in London. Summary box What is already known on this topic All-cause mortality during the COVID-19 pandemic was higher than in previous years; this excess mortality was particularly pronounced among elderly people, males, people of non-white ethnicity, people of lower socio-economic status and people living in care-homes. Several other papers have studied a wider range of factors associated with mortality due to COVID-19 using cause-of-death data. There is little evidence on how all-cause mortality has changed in people with comorbidities. What this study adds Our study shows that during Wave 1 of the pandemic all cause death rates increased by a similar proportional degree for almost all population subgroups regardless of their health or socio-demographic circumstances; the exceptions were those with a diagnosis of dementia or learning disabilities and those of non-white ethnicity or living in London. This suggests that COVID-19 has dialled up the risk of death by a similar proportional degree for everyone except those exposed to a higher risk of infection.