Background: Children with developmental language disorder (DLD) have problems acquiring language, affecting their communicative participation, social--emotional functioning (SEF) and quality of life (QoL). Aims: To investigate whether communicative participation mediates the relation between language and SEF and QoL. Methods & Procedures: In a longitudinal design, 511 children were recruited via early intervention groups for children with (presumed) DLD. Language and IQ scores were obtained at a mean age of 3;11 (T0). In kindergarten, communicative participation, SEF and QoL were measured via parental questionnaires (T1, mean age 4;8). The relationship between language and SEF and QoL was investigated directly and with communicative participation as a mediating factor using structural equation modelling. Outcomes & Results: Expressive grammar was related to communicative participation, SEF and QoL, while receptive language and expressive vocabulary were not. Children with better expressive grammar at T0 showed better communicative participation at T1. Better communicative participation, in turn, was related to less problems in SEF and higher QoL. We also found an unexpected positive direct relation between expressive grammar and problems in SEF. Post-hoc analyses showed that this was likely to be a suppressor effect, caused by a small subset of children with relatively good expressive grammar and poor communicative participation. Conclusions & Implications: Communicative participation is a mediator in the relation between language and SEF and QoL. These results underline the importance of addressing communicative participation as a functional measure of language ability both in research and clinical practice.