The son of a Royal Irish Artillery officer, Armstrong studied art in Dublin and served his apprenticeship as an engineer on the Irish and English railways before emigrating to Toronto in 1851. He was partner in the firm of Armstrong, Beer & Hime Civil Engineers, Draughtsmen and Photographers. He travelled to Lake Superior in 1859 and from then on took advantage of surveying work to travel and sketch extensively in lands then unsettled by Europeans. He took particular interest in the native populations of the northern plains, such as the Assiniboin and Sioux, glimpsed in the years immediately prior to their confinement to reservations.
Armstrong's sketches from his travels in 1859 to Fort William (now the city of Thunder Bay) were worked up into watercolours and presented to the Prince of Wales on his visit to Toronto in September 1860 (for which see D.Millar, The Victorian Watercolours and Drawings in the Collection of Her Majesty the Queen, I, London, 1995, cat. nos. 134-166). These drawings at Windsor include similar subjects to the present and following lot (no. 161 'Halt on White horse plain. Red river' and no. 162 'Buffalo hunters Camp. Red River'), suggesting the present watercolours also take their subjects from sketches taken in 1859 between Lake Superior and Winnipeg.