Adults with intellectual or developmental disabilities often have hypertension and mental illness, and are prescribed medications for treatment. This study examined psychotropic medication adherence as a mediator between the association of residence type and antihypertensive medication adherence for adults with intellectual or developmental disabilities. We used Medicaid data of adults with intellectual or developmental disabilities who had hypertension and prescribed antihypertensive medication (N = 1,201) to measure the direct effect, indirect effect, and total effect of residence type (home vs. supervised setting) and antihypertensive medication adherence, with a mediator of psychotropic medication adherence. The indirect effect of psychotropic medication adherence on antihypertensive medication adherence was 1.26 (OR = 1.26, CI: 1.08–1.52), holding residency constant. The direct effect of residential type on antihypertensive medication adherence was 3.75 (OR = 3.75, CI: 1.61–8.75). This association may be due to some features of having a mental illness or maybe the result of being prescribed more than one medication.