Abstract: The companions in crime hypothesis suggests that co-offending moderates the link between peer delinquency and adolescent delinquency. However, this hypothesis has rarely been investigated longitudinally. Hence, this study investigated the co-development of friends’ delinquency and adolescents’ delinquency, as well as the co-development of friends’ delinquency and short-term mindsets (impulsivity and lack of school future orientation). Whether this co-development is stronger when adolescents engage in co-offending was also investigated. Three data waves with two year lags from an ethnically-diverse adolescent sample (at wave 1: N = 1365; 48.6% female; Mage = 13.67; age range = 12.33–15.09 years) in Switzerland were used. The results from parallel process latent growth modeling showed that the co-development between friends’ delinquency and adolescents’ delinquency was stronger when adolescents engaged in co-offending. Thus co-offending likely provides direct access to a setting in which adolescents continue to model the delinquency they learned with their peers.