Illegitimate tasks are tasks that violate norms about what an employee can reasonably be expected to do. Representing a relatively recent stressor concept, illegitimate tasks have been linked to strain, but so far have been assessed only by self-report. The current multisource study investigates to what extent supervisors’ assessments of illegitimate tasks converge with incumbents’ self-reports of illegitimate tasks and predict three kinds of strain, namely psychological strain (incumbent report of exhaustion), behavioural strain (supervisors report of incivility), and family strain (partner report of work-family conflict). Low convergence between assessments was expected due to idiosyncratic appraisals but also to differing perspectives of supervisors and incumbents due to their roles, as described by the newly developed roles-as-perspectives theory proposed in this paper. Data from 166 triads were analysed by structural equation modelling and Relative Weight Analysis. Results showed that convergence between incumbent and supervisor reports of illegitimate tasks was rather low; it was higher when the supervisor had a limited span of control. Illegitimate tasks were associated with all three types of strain for both self- and supervisor-reports of illegitimate tasks, indicating that the detrimental effects of illegitimate tasks cannot be explained by common method biases alone and that incumbents and supervisors have overlapping but not identical concepts of illegitimate tasks.