Conventional expressions, a subset of multiword units, are the target of the current study, which aims to address questions concerning native and nonnative speakers’ knowledge and processing of a set of such strings. To this end, 13 expressions identified as conventional in the southwest of France were tested in an online contextualized naturalness judgment task, which was administered to 20 French natives, 20 long-stay (i.e., >1 year in the southwest of France) Anglophone nonnative speakers of French, and 20 short-stay (i.e., 4–6 months in the same region) Anglophones. The naturalness judgments provided by the participants revealed that all groups judged the conventional expressions similarly and significantly differently from the matched conditions, which involved grammatical but not conventional strings. The reaction time results suggested that conventional expressions have a mental correlate for both natives and nonnatives, although the processing patterns recorded differed for the two groups. The reaction time results are argued to be most consistent with a pragmatic competence model of conventional expression processing.