This research explores intralinguistic communicative mediation in an English for Specific Purposes (ESP) online journal writing to enhance professional development through the co-construction of meaning. It accentuates research gaps specific to learning English in the field of law under the approach of collaborative Computer Assisted Language Learning (CALL) and the Vygotskian pedagogical model of semiotic mediation with tools. The online journal writing allowed learners not only to engage in the discussions but to develop cognitive messages expressed at levels B1+/B2 of the "Common European Framework of References for Languages (CEFR)." The 69 participants were ESP students of "English Law," a compulsory subject in a professional accredited university master's degree in law, needed to work in Spain. The findings revealed that the three main strategies for knowledge building and intraliguistic communicative mediation were: linking to previous knowledge (class content or other), adapting the language, and breaking down complicated information. Moreover, the collaborative journal writing reinforced students' accuracy on expert vocabulary learning through purpose-driven mediation of texts, concepts, and communication in the context of law. Further results indicated that collaborative journal writing was an effective pedagogical tool to make meaning for ESP through different interactive affordances of mediation (linguistic, cultural, social, pedagogical, and professional) by comparing legal contexts from Spain, the United Kingdom and United States.