BACKGROUND: The COVID-19 pandemic has disproportionately affected socially vulnerable communities. Some individuals experience persistent symptoms and conditions of COVID-19 illness known as long COVID. As little research has examined how social vulnerability is related to long COVID, we studied this topic using Minority Health Social Vulnerability Index (MHSVI), specifically created for the COVID-19 pandemic in the U.S. METHODS: We merged county-level MHSVI data with population-based data of Michigan adults with PCR-confirmed SARS-CoV-2 infection between March 2020 and May 2022 based on respondents' county of residence. We examined the relationship between county-level MHSVI (binary: high social vulnerability ≥ 75th percentile) and two long COVID measurements, assessed a median of 18.8 months after their initial infection: (1) ongoing long COVID (yes/no) and (2) long COVID diagnosis (yes/no). We conducted modified Poisson regression models with robust standard errors to estimate prevalence ratio (PR) between associations of MHSVI and long COVID overall and by six MHSVI themes (socioeconomic status, household composition/disability, minority/language, housing type/transportation, healthcare access, medical vulnerability), adjusting for individual-level and county-level covariates. RESULTS: Living in high MHSVI counties was not associated with ongoing long COVID or long COVID diagnosis. However, the associations differed by theme of MHSVI: respondents in highly socially vulnerable counties assessed by medical vulnerability had 1.32 times higher prevalence of long COVID diagnosis (95% CI:1.12 - 1.57). There were no statistically significant associations in other themes after the adjustment for covariates. CONCLUSIONS: Our findings suggest the importance of upstream social determinants of health during public health emergencies and provide evidence that medically vulnerable communities need additional public health resources to cope with long COVID among their residents.