Served as a medical missionary in Rajputana, India for 13 years before returning to Scotland in ill health. Following his return he campaigned for funds to establish a medical missionary training establishment in India, and made speeches to raise money for the project in Manchester, Liverpool, Glasgow, Edinburgh, Abderdeen and London.
He returned to India in 1876 and in 1877 purchased a property previously owned by the American Presbyterian Church for 8,600 rupees, though delays in staffing prevented the institution from opening until 1881, as the "Agra Institute" with Valentine as its superintendent. The first student, Henry Phillips, stayed on as his assistant. Scholarships were provided in conjunction with the Agra Medical College, with additional classes for women involved in "zenana missions" to receive basic medical education.
In 1883 his health deteriorated once more and he returned to Scotland. During this time the decision was made to amalgamate the Institute with the Edinburgh Medical Missionary Society, which came to pass in July 1885, becoming the Agra Medical Missionary Training Institute.
He retired from missionary work in 1901 and died in 1905.
