Purpose: To investigate changes in healthcare workers’ mental-health under prolonging Covid-19 pandemic conditions.Methods: A monthly survey over a full year was conducted for employees of the HUS Helsinki University Hospital (n = 4804) between 4th June 2020 to 28th May 2021. Pandemic-related potentially traumatic events (PTEs), work characteristics (e.g., contact to Covid-19 patients), and other covariates were used to predict Mental Health Index-5 (MHI-5) and Insomnia Severity Index (ISI) in generalized multilevel and latent-class mixed model regressions.Results: Local Covid-19 log-incidence (odds ratio, OR = 1.21, with 95% CI = 1.10–1.60), directly caring for Covid-19 patients (OR = 1.33, CI = 1.10–1.60) and PTEs (OR = 4.57, CI = 3.85–5.43) were all independently associated with low mental health, when (additionally) adjusting for age, sex, profession, and calendar time (a 5th degree polynomial expansion). Independence from time suggests effects of incidence change in time. Effects of local Covid-19 incidence on sleep were fully dependent on time. Latent mental-health trajectories were characterized by a large class of “stable mental health” and minority classes for “early shock, improving” and “early resilience, deteriorating” mental health. The minority classes, especially “early shock, improving”, were more likely to live alone and be exposed to PTEs than others.Conclusion: Healthcare workers face increasingly heterogeneous mental-health challenges as the Covid-19 pandemic prolongs. Adversity and mental ill-being may accumulate in some employees. More research is needed on the factors affecting employees’ resilience to the prolonging pandemic. Living arrangements may play a role.