During World War II, Joslin served in the Navy in the Pacific Theater and graduated from Columbia Law School in 1947. He married Mary Coker in 1946 and the couple moved back to Raleigh in 1948. In 1951, they purchased the land that would later become Joslin Gardens.
Joslin's life and career are characterized by his progressive views and his dedication to the preservation of natural areas. He led a Nature Conservancy campaign to create the Recreation and Natural Heritage Trust, a dedicated fund for acquiring and managing ecologically significant land across the state. He was also active in the work of the Triangle Land Conservancy. In 1990, Joslin received the national Nature Conservancy’s highest honor for volunteer service, the Oak Leaf Award.