After education in the schools at Charleston, Mount Forest, and Guelph he engaged in business, but a religious awakening led him to determine to devote his life to medical missionary service. Accordingly he entered the University Medical College, New York City, where he was graduated in 1889. During his student days as one of the early members of the Student Volunteer Movement for Foreign Missions he was active in awakening missionary interest by addresses and by a unique pamphlet, The Medical Mission, Its Place, Power and Appeal (1911), which was the forerunner of missionary literature of its type.
On April 1, 1889, he was appointed a missionary of the Presbyterian Board of Foreign Missions, and was enabled to sail for India the following fall by special contributions from the Bryn Mawr (Pa. ) Presbyterian Church, which supported him during his entire missionary career. He began work at Sangli with a small dispensary, his equipment being improvised from packing boxes. In 1892, however, he moved to Miraj, where the prime minister of the state, who had been one of his patients, gave land for a hospital, which was erected by gifts from John H. Converse, president of the Baldwin Locomotive Works. The Maharajah of Kolhapur, also a patient, became a devoted friend and munificent benefactor and was deeply influenced in his anti-caste attitude by the democracy and brotherhood which he saw in the hospital. From the beginning Wanless' skill and ability drew ever increasing patronage and support, and with funds received from his patients and other friends he developed the most extensive and effective medical missionary plant in India. He established, also, in 1897, the first missionary medical school in India and, in 1900, a leper asylum. A tuberculosis sanitarium, which now bears his name, was projected by him and opened for patients in 1931. From all over southern Asia patients came to Miraj, attracted by his fame, and his medical students were to be found throughout India and far up into Mesopotamia. During his superintendency approximately a million patients passed through the hospitals, and he himself performed annually some 6, 000 operations. He was as earnest and thorough an evangelist as he was a skillful and efficient physician and surgeon. Wanless was thrice decorated by the government of India. In 1910 he received the Kaiseri-Hind Medal, Second Class; in 1920 he was awarded the Kaiser-i-Hind Medal, First Class; and in 1928 he was knighted for his extraordinary service. The only American to receive this last honor previously was J. C. R. Ewing, who was for forty-three years a missionary in India. In 1928 ill health compelled Wanless to retire to America, but he was called back to India in 1930, where he remained some months, aiding in the establishment of the tuberculosis sanitarium. He was a vigorous personality, overflowing with energy and good cheer. A remarkable testimony to his place in public esteem was a great meeting of representatives of all communities, held in Poona in January 1928, under the chairmanship of the Aga Khan, for the presentation of a farewell address enclosed in a silver casket. He died in Glendale, Cal. Some account of his activities is given in his book, An American Doctor at Work in India, which appeared in 1932.
Wanless was married twice: on September 5, 1889, to Mary Elizabeth Marshall, who died August 12, 1906; and on December 5, 1907, to Lillian Emery Havens, who survived him.