The present research used a longitudinal framework to test a model of willingness to communicate (L2WTC), informal digital learning of English (IDLE), and foreign language boredom (FLB) in learning English as a foreign language among 325 Iranian university students attending a course in general English for 4 months via a latent change score mediation (LCSM) model. Overall, model indices showed an acceptable model fit for the LCSM model. There was a significant and positive constant growth in IDLE and L2WTC, a negative constant growth in FLB across four measurements, while the proportional change was significant and negative for all three variables. The significant interindividual and intraindividual differences in patterns of growth across time were found for the three variables. Also, moderate and positive cross-system associations were revealed between IDLE and L2WTC across four measurements. Moreover, moderate and negative cross-system associations were revealed between FLB and L2WTC as well as between IDLE and L2WTC. Furthermore, changes in FLB mediated subsequent associations between changes in IDLE and L2WTC. The results highlight the pivotal role of variations in FLB as a mediator for the longitudinal link between IDLE and L2WTC.