The COVID-19 pandemic has provided a collective opportunity to engage in prosocial behaviours, including kindness; however, little is known about the long-term impacts of the pandemic on such behaviours. As a part of a larger study (Health Outcomes for Adults During and Following the COVID-19 Pandemic), the purpose of this mixed methods research was two-fold: (1) to quantitatively explore adults’ prosocial behaviour over time during the first 16 months of the pandemic in Ontario, Canada (April 2020-August 2021); and, (2) to more deeply explore, via focus groups, a sub-sample of Ontario adults’ lived experiences of prosocial behaviour (assessed March 2022). At three time points, participants (N = 2,188) completed online questionnaires which included demographics, Prosocialness Scale for Adults, and three global kindness questions. A subset of participants (n = 42) also participated in one of six focus groups exploring their experiences of prosocial behaviour during the pandemic. A series of one-way repeated measures ANOVAs revealed that participants’ self-reported prosocial behaviour increased significantly over time, while participants’ awareness of kindness, engagement in acts of kindness, and view of kindness as crucial significantly decreased. Thematic analysis revealed three main themes: (1) shift in prosocial behaviour during the pandemic; (2) kindness from various perspectives; and (3) prosocial burnout. This study provides insight into the longer-term effects of the pandemic on adults’ prosocial behaviours and should be leveraged to help understand how individuals respond in times of crises.