Despite near universal health coverage under Medicare, racial disparities persist in the treatment of diffuse large B-cell lymphoma (DLBCL) among older patients in the United States. Studies evaluating DLBCL outcomes often treat socioeconomic status (SES) measures as confounders, potentially introducing biases when SES factors are mediators of disparities in cancer treatment.To examine differences in DLBCL treatment, we performed causal mediation analyses of SES measures, including: metropolitan statistical area (MSA) of residence; census-tract poverty level; and private Medicare supplementation using the Surveillance, Epidemiology and End Results-Medicare linked database between 2001 and 2011. In this retrospective cohort study of DLBCL patients ages 66+ years, we conducted a series of multivariable logistic regression analyses estimating odds ratios (OR) and 95% confidence intervals (CI) relating chemo- and/or immuno-therapy treatment and each SES measure, comparing non-Hispanic (NH)-black, Hispanic/Latino, and Asian/Pacific Islander (API) to NH-white patients.Compared to NH-white patients, racial/ethnic minority patients had lower odds of receiving chemo- and/or immuno-therapy treatment (NH-black: OR 0.84, 95% CI 0.65, 1.08; API: OR 0.80, 95% CI 0.64, 1.01; Hispanic/Latino: OR 0.78, 95% CI 0.64, 0.96) and higher odds of lacking private Medicare supplementation and residence within an urban MSA and poor census tracts. Adjustment for SES measures as confounders nullified observed racial differences. In causal mediation analyses, between 31% and 38% of race/ethnicity differences were mediated by having private Medicare supplementation.Providing equitable access to Medicare supplementation may reduce disparities in receipt of chemo- and/or immuno-therapy treatment in older DLBCL patients.