This study reports the results of a mixed methods approach to investigate the impact of instructor feedback in mobile-assisted language learning (MALL) on the grammatical development of English as a foreign language (EFL) learners. The participants of this quasi-experimental study included 40 female EFL learners randomly selected from the English learning chat groups on Telegram, an online instant messaging application. Dialang, a free web-based language proficiency test, was used to assess the learners' grammatical knowledge as pre-test on the basis of which the participants in the experimental group were categorized into 15 chat groups, each including the instructor and two participants of the same level of proficiency in English grammar. A paired samples t-test was utilized to analyze the quantitative data. The results indicated that using chats in Telegram led to a significant development in learners' grammatical accuracy in the experimental groups. Semi-structured interviews were conducted to explore the learners' attitudes towards their experience in MALL immersion program. The thematic analysis used to analyze the qualitative data revealed a number of themes which addressed the learners' positive perceptions towards using MALL applications to develop their grammatical structures in online chats.