He transferred to the Royal Flying Corps in 1913 and during the First World War served as a squadron leader and wing commander before taking up senior training and staff appointments.

In April 1918 he transferred to the newly formed Royal Air Force and was subsequently promoted to brigadier general just after the end of the First World War.

When the First World War began, MacLean was an instructor at the Central Flying School, holding the rank of captain.

With the establishment of the Royal Air Force on 1 April 1918, MacLean became one of the new Service's senior officers.

Just after the close of the Great War, MacLean was appointed brigadier general responsible for administration at the headquarters of the RAF's South-Eastern Area.