Compared to the high number of studies that investigated executive functions (EF) in children with attention-deficit/hyperactivity disorder (ADHD), a little is known about the EF performance of adults with ADHD. This study compared 37 adults with ADHD (ADHDtotal) and 32 control participants who were equivalent in age, intelligence quotient (IQ), sex, and years of education, in two domains of EF—set shifting and working memory. Additionally, the ADHDtotal group was subdivided into two subgroups: ADHD patients without comorbidity (ADHD−, n = 19) and patients with at least one comorbid disorder (ADHD+, n = 18). Participants fulfilled two measures for set shifting (i.e., the trail making test, TMT and a computerized card sorting test, CKV) and one measure for working memory (i.e., digit span test, DS). Compared to the control group the ADHDtotal group displayed deficits in set shifting and working memory. The differences between the groups were of medium-to-large effect size (TMT: d = 0.48; DS: d = 0.51; CKV: d = 0.74). The subgroup comparison of the ADHD+ group and the ADHD− group revealed a poorer performance in general information processing speed for the ADHD+ group. With regard to set shifting and working memory, no significant differences could be found between the two subgroups. These results suggest that the deficits of the ADHDtotal group are attributable to ADHD rather than to comorbidity. An influence of comorbidity, however, could not be completely ruled out as there was a trend of a poorer performance in the ADHD+ group on some of the outcome measures.