Social isolation is a relevant problem for veterans who are at risk for disengaging from others as a function of transition stress from military life to civilian life, and given high rates of exposure to trauma and psychological distress. Few researchers have examined social isolation in veterans over time, particularly during COVID-19 that led to significant barriers and restrictions on social interactions. The purpose of this longitudinal study was to assess veterans’ experience of social isolation and its mental health and social functioning correlates during a 6-month period of the COVID-19 pandemic. Participants were 188 United States veterans of the Iraq and Afghanistan wars, who completed a total of four assessments: one every two months for a total duration of six months. Surveys included measures of global mental health and social functioning as indicated by perceived emotional support, quality of marriage, and couple satisfaction. Multilevel modeling was used to assess 1) growth models to determine whether social isolation changed over time and the trajectory of that change (i.e., linear or quadratic); and 2) whether social isolation was related to both concurrent and prospective indicators of mental health and social functioning. All analyses included person mean centered and grand mean centered isolation to assess for within-and between-person effects. Veterans reported a quadratic trajectory in social isolation that decreased slightly and stabilized over time. Findings indicate that higher social isolation, at both the within- and between-person level, was negatively associated with concurrent emotional support, mental health, quality of marriage, and couple satisfaction. However, all prospective effects were nonsignificant at the within-person level. Results suggest although isolation may decrease over time, veterans report worse mental health and social functioning during times when they report higher levels of social isolation compared to themselves and others. Future work is needed to determine if interventions can be applied during those times to prevent or target those negative associations.