Psychological capital (PsyCap) has gained prominence as an important resource for positive work attitudes, behaviors, and organizational outcomes. This pre-registered study aims to broaden existing understanding of the relationship between PsyCap and positive attitudes and behaviors using longitudinal evidence. A sample of 202 teachers (M = 45.33 years, SD = 10.76) completed a set of online questionnaires in two measurement waves, two years apart. Using structural equation modelling with a pre-registered syntax, we found support for PsyCap as a mediator of the effects of perceived social support on changes in work engagement and life satisfaction within the two-year period. Perceived social support predicted the level of PsyCap measured two years later. A higher level of PsyCap was positively associated with changes in work engagement and life satisfaction between the two measurement waves. As the first data collection took place in the spring of 2019 and the second in the spring of 2021, the results also highlight the role of social support and PsyCap in dealing with demands related to the COVID-19 pandemic.