Preoccupation of criminologists with juvenile-onset criminal careers overshadows the fact that offenders who begin their criminal careers in adulthood comprise a substantial portion of adult offender populations. Little is known about adult-onset offenders, generally, and even less about first-time adult-onset offenders. Using a large sample of adult felons on probation supervision, this study explores differences between first and repeat offenders. With respect to risk factors at intake, timing of rearrest, and frequency and nature of supervision failures over 3 years, first-time adult-onset participants exhibited statistically significant differences in relation to both repeat adult-onset and juvenile persistent offenders, with largest differences occurring in analyses involving the latter. With respect to risk factors at intake and rearrest, events in adulthood played a more dominant role among first-time adult-onset offenders compared with other groups, where criminal lifestyle factors were in greater evidence. The article concludes with a discussion of community supervision practices to prevent the progression of the first-time adult-onset offender's criminal career, social reforms to assist this group, and avenues for relevant future research. (Published Abstract)