Teachers' professional development plays a significant role in the sustainability of quality education. However, providing sustainable professional development programs is difficult because of various challenges, such as insufficient government policies, a scarcity of qualified personnel and training opportunities, lack of access and resources, etc. The COVID-19 epidemic has further intensified the problem and adversely harmed schoolteachers’ interest, motivation, and involvement. In such situations, technology can play a positive role in carrying out sustainable educational activities. However, little focus has been paid to the use of technology for the sustainable professional development of teachers, particularly in the under developing countries. Due to its unique qualities, mobile learning is receiving much attention as an essential component to impart sustainable education to teaching professionals. It represents a paradigm shift in ongoing professional development, particularly for teachers who are constrained by time, resources, and travel requirements. Mobile learning is an alternate and complementary technique to enhance teachers' professional development, allowing them to work, learn, and attain their full potential. To meet the sustainable development goal of teachers’ education, in [1] we have provided a framework for teachers' continuous professional development (CPD). Based on that, we have also developed a mobile App to provide training and ongoing support to teachers’ professionals. This study evaluates the usability of our mobile App for the sustainable development of teachers. Usability characteristics of usefulness, efficiency, and satisfaction are measured using a usability testing approach. 107 elementary school instructors took part in the study. Participants in the study performed eight usability tasks in the same room using the researcher's internet bandwidth. The participants' time and completion rate were recorded. A post-test questionnaire followed usability tasks. After the usability activities, participants completed a System Usability Scale questionnaire to assess overall satisfaction with the training app and gain a broad picture of the program. The application achieved a 100% completion rate for all tasks, indicating an extremely effective, efficient, and sustainable system. Moreover, the satisfaction percentage was also found to be 84.1 percent, which is considered highly satisfactory.