Hi,
Missionary work is an interesting case- one could view it as analogous in some ways to both intense volunteer activity and working for a nonprofit. It is certainly a valuable way of showing dedication to a cause and international experience. BYU considers missionary experience as a good means of demonstrating leadership skills:
"Full-time management work experience is strongly recommended but not required for admission to the MBA program; evidence of management and leadership skills is required. These skills are evidenced through completion of the MBA application (essay question responses, resume, letters of recommendations, etc.), work experience, LDS mission experience (if applicable), and an interview with a member of the admissions committee (interviews are conducted by invitation only, and are offered to applicants the admissions committee deems competitive for admission to the program after review of the completed application)."
Texas:
"What is full-time work experience?
Full-time work experience is calculated up until the time of enrollment (start of fall classes). This includes all full-time work that you have completed since receiving your bachelor’s degree. This does not need to be business-based, and can include teaching, military service, government experience, programs like Peace Corps, etc. The average number of years of work experience for our admitted class is five years, but our minimum for admission consideration is two. If you have other SUBSTANTIAL experience and would like to be considered for admission, please submit an application and explain your experiences and circumstances fully in your essays."
https://mba.mccombs.utexas.edu/admissions/adm/faq.asp