Increasingly, interventions are being developed to promote collaboration across health care and social service (such as food, housing, and transportation) sectors. During the COVID-19 pandemic, demand for social services grew while social service organizations' capacity declined due to constraints on staffing, funding, and operations. We used an organizational survey fielded from July through November 2020 and publicly available, county-level data to assess the pandemic's impact on 253 social service organizations in the Accountable Health Communities Model evaluation. Over half of surveyed organizations reported being severely impacted by the pandemic, and 92% reported being at least moderately impacted. Social service organizations without federal funding and those in counties with lower poverty (smaller proportion of residents in poverty) and higher COVID-19 case rates were most impacted by the pandemic. Understanding the pandemic's burden on social service organizations can inform planning for future collaborations across health care and social service sectors.