Over one-third of the inmates (39 percent) reported that a mentor introduced them into a criminal environment. The presence of a mentor was significant for achievement in the protégés criminal career. Moreover, mentors indirectly improved criminal gains and directly lessened the costs of their protégés criminal activities. Mentors tended to emerge during the late adolescence and early adulthood period when most offenders were becoming less involved in crime and are more likely to disengage from criminal activity. Mentors were, on average, 11.4 years older than the protégés. The analysis focuses on an integrated strain theory, social control theory, and learning theory of deviance that, taken together, suggest a lack of parental supervision provides the opportunity to meet and be co-opted by accomplished offenders. Moreover, conventional mentorship research indicates that unsupervised youth will eventually be informally supervised by mentors outside of their family, which in this case may be criminal mentors. Participants were 268 male inmates incarcerated in 5 federal prisons in Quebec, Canada who responded to face-to-face interviews typically lasting approximately 2 hours. Interviews focused on inmates’ backgrounds, criminal activities, criminal contacts, entourages, and earnings during the 3 years prior to incarceration. Data analysis techniques included linear regression and path analyses. Future research should focus on the possible negative consequences emerging from criminal mentor-protégé relationships. Footnotes, tables, figure, references