At the age of thirteen Mary moved with her family to Huron, South Dakota. Her father was in the lumber business there and later served as superintendent of schools. After years of education at several schools and some teaching, she was appointed as an overseas missionary. Her responsibilities during her first term of mission work in Balasore, India (1925) included teaching language in high school, and also required her to be "parents" to many of the girls that were orphans.

After 1932 she was assigned to Burma and was one of the last missionaries to leave as World War II heated up. She writes that the trauma of having to send the girls home and close the school when the Japanese invaded Burma was difficult. She also recalls watching Japanese planes being shot down, spending nights in a trench, and finally taking the last train for Mandalay. She was then forced by Chinese troops to flee to China and landed in Assam, India. She spent the remaining war years helping the YWCA, and proof reading the Kachin New Testament being reprinted by the British Foreign Mission Society. During her war experiences, she lost a great deal of weight and suffered for over a year with malaria.

After spending a brief time in 1946 with her parents in Huron, she returned with her missionary companion, Lucy Bonney, to Burma. They arrived in 1947. An article in the Huron Plainsman during that time was entitled "To Dispense Religion from a Jeep", as jeeps were used to traverse the rough mountain roads in Burma. While Laughlin was primarily an English teacher, in her courses she taught subjects ranging from accounting to piano, and she especially enjoyed working with chorus and glee clubs.

After retiring from her mission post in 1964, she found deep satisfaction in knowing that half of the school's staff was composed of former students who'd returned to serve in a Christian school. The next 30 years of retirement was spent in a Baptist Home in California where she was very active in a variety of roles. For example, as long as her health permitted she could be found in the dining room feeding those who could no longer feed themselves. During this time, after noting in a letter she wrote describing her many physical problems, she remarked "So you see I have little to complain about."

Her dedication to missions was shown in a letter written to her family in 1942 which said "It may be many, many more months before I can write to you but I know God will be caring for me, and that I am here where He wants me to be." This sums up the life and service of Mary Irene Laughlin, serving in Christ's name, at home and abroad, in wartime and peacetime