Walker's first military service was in 1866 when at age 18 he took a three-month officer training course in Toronto conducted by the 2nd Battalion of the
17th Regiment of Foot. In 1871 he served with the
37th (Haldimand) Battalion of Rifles during the
second Fenian campaign. Walker's reasonable treatment of the First Nations population gained him disfavour in Ottawa but was instrumental in keeping the peace between those peoples and the white settlers. Some criticized him for supplying food to hungry indigenous people in advance of their official rations. The natives referred to him as Pee-tee-quack-kee, "the Eagle that protects."
Later service During the
North-West Rebellion of 1885 Walker commanded a home guard in Calgary. In 1905 Walker created the
15th Light Horse regiment, and commanded it with the rank of lieutenant-colonel. In 1911, Walker became the Honorary Lieutenant Colonel of the 23rd Alberta Rangers. During the First World War, at the age of 70, Walker went overseas as a captain in the 238th Forestry Battalion (a unit of the
Canadian Forestry Corps) and was later promoted to major. He returned to Calgary in 1919. ==Civic duty==