Horsfield began the initial clearance and conservation of Jerash in 1925, and excavated at Petra with his future wife, Agnes Conway in 1929.
He attended Leeds Grammar School and moved to London to train in architecture in the office of noted Gothic architect George Frederick Bodley.
Horsfield then moved to the United States to work for the architectural firm Cram, Goodhue & Ferguson.
He saw action in the Gallipoli campaign in 1915, after which he was commissioned into the 7th West Yorkshire Regiment and took part in the Battle of the Somme in 1916.
After contracting trench fever he was posted to India in 1918, attached to the Royal Engineers, and became the Chief Architect for Military Works, Rawalpindi, and Simla.