This research endeavors to assess the efficacy of Mobile Assisted Language Learning (MALL) in elevating students' English proficiency to the Common European Framework of Reference (CEFR) B1 level. The study involves identifying the most suitable mobile application for formative assessment, implementing it to enhance students' English competency, and investigating their attitudes toward MALL in formative assessment. Engaging four specialists, a thorough evaluation was conducted to determine the most effective mobile application, revealing Quizziz as the optimal choice for formative assessment. The study involved a purposively selected sample group comprising 139 undergraduate students from a private university, in Thailand, enrolled in the English at Work Course. This sample group underwent pre-tests, post-tests, and self-evaluation using the CEFR B1 abilities checklist and provided feedback through questionnaires and in-depth interviews to gauge their perception. Data analysis involved various statistical methods, including t-test dependent, one-way ANOVA, repeated measures ANOVA, mean, and standard deviation. Results indicated a statistically significant improvement in post-test scores compared to pre-test levels, with the fourth assessment revealing the highest mean score ([x-bar] = 9.11, SD = 3.12). Notably, the mean scores for each competency descriptor at the CEFR B1 Level demonstrated considerable variation (F = 21.717, p < 0.001), indicating enhanced performance across competencies. According to the questionnaires, student perception was significantly high ([x-bar] = 4.64, SD = 0.47), affirming MALL's effectiveness as a formative assessment tool. The corresponding in-depth interview data further underscored students' positive feedback and favorable attitudes toward MALL. Therefore, this research not only contributes to the understanding of the effectiveness of MALL in enhancing English proficiency but also sheds light on the practical implications of integrating technology-driven formative assessment methods into language education. This study provides valuable insights for educators seeking to create engaging and effective language learning environments by elucidating the positive student attitudes and perceptions toward MALL.