We use longitudinal data from a representative sample of the UK and compare self-reported mental health, as measured by the GHQ-12 score, at three timepoints (2017–2019, April 2020 and March 2021), for the whole sample and by sex and ethnicity. Out of the 14,382 individuals interviewed in 2017–2019 and April 2020, 10,445 were interviewed again in March 2021. The mean GHQ-12 in April 2020 is 12.37 [95% CI: 12.22, 12.52] and in March 2021 is 12.36 [95% CI: 12.21, 12.51], above that of 2017–2019: 11.13 [95% CI: 10.99, 11.26]. We do not find evidence that the level of mental health goes back to pre-pandemic levels. In terms of inequalities, while the gender gap (mean difference between women and men) in mental health deterioration among White British is closing, there is no clear evidence that the ethnic gap (mean difference between ethnic minorities and White British) among men is changing.