Background Offending is known to be associated with various health problems and premature death, but previous studies on associations between offending and mortality have often been only with men, convicted offenders or highly selected samples. A more sensitive measure of offending may be preferable when trying to understand the extent of health disadvantages among people who offend. Aims The aim of this paper was to study the associations between investigated offending, death and causes of death in a nationally representative birth cohort. Methods A broad concept of offending was used such that people who had had any contact with the police because they had been suspected of crime were included. Offending data were obtained from the National Police Register for 5405 men and women born in Finland in 1981, spanning their ages 15–30 years; mortality data were received from Statistics Finland. Offending was classified into four categories by frequency: none, 1–4 different offence contacts, 5–27 and 28 or more. Causes of death were categorised into natural, accidents, suicide or homicide. Of the cohort, 2304 (43%) had offended and 57 (1.1%) had died. Associations between offending, mortality and causes of death were analysed, controlling for parental education level and family structure in childhood. Results The mortality rate was higher among offenders than non‐offenders, increasing with rising frequency of offending. The most frequent offenders were nearly 30 times more likely to have died by age 30 than non‐offenders (odds ratio 28.6, confidence interval 12.1–67.5); risk was higher for female than male offenders. Death among offenders was less likely to be from natural causes. Implications A heightened risk of premature death is the ultimate form of adverse outcome for offenders. Offenders' health is likely to need more attention as part of their overall management, for recidivists in particular. In our study, offenders' only certain criminal justice contact was with the police, so it may be that the police should take a greater role in signposting them towards health services. Offender treatment is needed not only to protect potential victims but also offenders themselves. Copyright © 2015 John Wiley & Sons, Ltd.