This systematic review examines the socio-economic and demographic characteristics of adults enrolled in US-based randomized controlled trials of mindfulness-based stress reduction (MBSR) and mindfulness-based cognitive therapy (MBCT). We conducted a literature search using Medline, PsycINFO, and CINAHL electronic databases from the earliest available until March 2017. We found 69 randomized controlled trials that met inclusion criteria. Only one study specifically aimed to test the mindfulness interventions in racial/ethnic minority or lower socio-economic status populations and none reported their effectiveness in these populations. For the 56 articles that reported the proportion of Caucasians in their samples, 79% of all individuals were identified as Caucasian. Out of the 45 studies that reported data on other races and ethnicities, 76% of the individuals were Caucasian, 11% were African-American, 4% were Asian-American, 4% were Hispanic/Latinx, < 1% were American Indian/Native American, and 5% were reported as “other” or multi-ethnic. The three most commonly reported socio-economic characteristics were educational attainment (n = 48), employment/professional status (n = 29), and income (n = 27). The proportion of college graduates in the study samples was higher than the national average in all but one of the studies that reported this data. Most of the studies that reported employment rates had more than half of their participants employed. Of the studies that reported average income, most had more than 50% of their participants making over $40,000 a year. This review highlights the need for increased diversity in randomized controlled trials of mindfulness-based interventions as these interventions have great promise to alleviate mental and physical health problems in underserved populations.