[3] Ellison joined the British Army in the early 1720s despite being his father's eldest son, because his family were in straitened circumstances due to debts accrued to Elizabeth's brother George Liddell.

[7] These periods of official leave were augmented with occasional visits home to recover his health at spas, Ellison suffering throughout his life from a weak constitution.

Having completed these tasks Ellison went in to winter quarters at Ghent, with his regiment having been severely depleted through bad weather, long marches and sickness.

[10] He continued on the continent after this and saw action at the Battle of Dettingen on 27 June 1743, where the army fought to relieve French pressure on their allied forces; by October his regiment only had seven healthy officers, including himself, with which to operate.

[2][11][9] Later in the year the colonel of the 23rd died of wounds he had received at Dettingen; while Ellison was his natural successor as lieutenant-colonel of the regiment, his position as adjutant-general meant he was bypassed because his duties were seen as too important.

[2][13] Ellison returned home from the continent in the summer of 1744 to recover at Bath from painful cramps and swellings in his limbs that had begun while on campaign; a hypochondriac, he attributed these difficulties to his weak constitution.

[12][13][14] Ellison was then promoted to colonel and served as adjutant-general on the staff of the Duke of Cumberland during the Jacobite rising of 1745, despite holding no regimental rank and receiving no pay for his efforts.

He was then advanced to lieutenant-general in 1759 under the express orders of George II, but Ellison found little interest in his promotion, writing that "the rank is of consequence to an able and healthful man but 'tis none to me who have been fitter for an hospital than the field".