Evidence-based smoking cessation treatment is safe and effective for smokers with serious mental illness (SMI); however, quit rates remain low in this population. We explored how social networks influence smoking outcomes among people with SMI who participated in smoking cessation treatment at community mental health centers. We conducted egocentric social network interviews with 41 individuals with SMI who participated in a statewide Medicaid demonstration project of smoking cessation treatment. We estimated the proportions of current smokers, former smokers, and never smokers in participants' (i.e., egos') networks and examined support for quitting from alters, defined as family, friends, peers, or significant others. We used logistic regression and mixed-effect logistic regression to explore the relationship between social network variables and ego's smoking status following cessation treatment. Egos reported an average of 5.9 ± 2.2 alters; 52% of alters were current smokers; 18% were former smokers; and 30% never smoked. The majority (57%) of alters helped an ego quit smoking. The presence of former smokers in the network was associated with decreased odds that the ego was smoking post-treatment. Egos whose friends were highly interconnected were less likely to smoke after treatment. Former smokers in the network may be a valuable resource for quitting particularly for vulnerable groups where there is a high prevalence and acceptability of smoking. Our initial findings suggest that a highly interconnected friendship network structure may be beneficial for people with SMI who are trying to quit smoking.