Previous observational research has indicated the effectiveness of a 12-step, dual-focus mutual aid group, Double Trouble in Recovery (DTR), for assisting individuals to recover from co-occurring substance use and psychiatric disorders. The current study extends this line of research by evaluating DTR with a quasi-experimental design; controlled designs are rare in studies of mutual aid. Patient outcomes in the same psychiatric day treatment program were compared for two consecutive admission cohorts characterized by high rates of co-occurring disorders. The first cohort did not have DTR available while the second cohort was exposed to DTR after it was established at the program. Both cohorts were assessed at program admission and at a six-month follow-up. Using intent to treat analysis, the Post-DTR cohort as compared with the Pre-DTR cohort had significantly fewer days of alcohol and drug use, more frequent traditional 12-step groups outside of the program and higher psychiatric medication adherence. There were no differences in psychiatric symptoms or program retention, however. This study helps demonstrate the benefits of introducing 12-step, dual-focus mutual aid into psychiatric treatment programs that serve patients with co-occurring disorders.