The purposes of this research were 1) to study the user experience and needs of EFL students and teachers in learning English writing skills; 2) t to design and develop a mobile-assisted language learning (MALL) application, teaching writing procedure, and writing activities; and 3) to implement a MALL application and, teaching writing procedure to students. The samples were 665 EFL lower secondary students in Thailand as the online survey respondents and five high school teachers as the interviewees. The experiment samples were 35 EFL lower secondary students. The research instruments consist of the MALL application, the learning process, English writing tasks, English writing tests, scoring writing rubrics, and a questionnaire on student satisfaction. The experimental period lasted for 8 weeks. The data were analysed by using frequency, mean, standard deviation, priority needs index, dependent t-test, and one-way ANOVA with repeated measure analysis. The research result indicated that 1) lower secondary students were able to use the internet on a regular basis. Students wanted to learn writing skills relevant to the student's experience through the study of writing errors, writing samples, writing styles, peer assessment, and constructive feedback from teachers. 2) The learning process of mobile-assisted language learning application using a task-based approach and gamification to enhance lower secondary students' English writing skills consisted of 3 components: pre-task, task process, and language focus. The 7 steps are as follows: selecting themes, exploring topics, drafting ideas, editing tasks, giving feedback, reviewing submissions, and concluding findings. 3) The experiment results indicated that the samples' English writing skills in the post-test were higher than in the pre-test at the 0.05 level of significance. 4) The difference in mean scores in the English writing tasks showed that there was a difference between English writing scores at the 0.05 level of significance (F=1579.266, sig=0.000). 5) The result of satisfaction showed that the respondents were very satisfied with the learning process and MALL application.