He rose to command the 79th Regiment of Foot before serving as second-in-command of the British Auxiliary Legion during the First Carlist War.

After initial service at the Cape of Good Hope he fought through much of the Peninsular War in Spain and Portugal as a subaltern, being seriously wounded at the Battle of Salamanca.

He became aide de camp to Major-General Robert Ross during the War of 1812, and was by his side at the Battle of Baltimore when the general was killed.

[1] MacDougall transferred to the 85th Regiment of Foot on 25 January the following year, then participating in the Siege of San Sebastián from July until its capture on 31 August.

He was riding with Ross, the latter returning from reconnoitring the frontline, when an American soldier hiding behind a tree shot and mortally wounded the general.

He fell from his horse, being caught by MacDougall and Captain Edward Crofton of the Royal Navy, and died three hours later.

[1] As British troops began to flee from the assault on the American defences, Pakenham rode forward to stop the retreat, with MacDougall by his side.

[1][11] MacDougall subsequently provided evidence as a witness to the court-martial of Lieutenant-Colonel Thomas Mullins, who had forgotten to take ladders with his regiment for the assault at New Orleans and was cashiered.

[14] MacDougall was promoted to brevet lieutenant-colonel on 25 April 1825 and given the responsibility of organizing the colonial militia in Nova Scotia.

With the 79th in Canada, he assumed command of the regiment in 1832 and provided military aid to the authorities in Montreal by putting down civil disturbances.

[16] Commanded by De Lacy Evans, who had served with MacDougall at New Orleans, the force would fight in the First Carlist War in support of Isabella II.

In an early morning frontal assault the Carlist besieging force was pushed away, costing the Legion 131 men killed and 709 wounded.

While the action, the Battle of Lugariz, was victorious, officers criticised Evans for not waiting for his reinforcements and for persevering with costly frontal assaults.

[18] By June discontent in the Legion was mounting again; pay was in short supply and the officers were frustrated with an order from Evans that made the Royal Marines they were fighting alongside outrank them.

[19] After Lugariz Evans chose to remain outside of San Sebastián, believing that to advance further would be to allow Carlist forces the opportunity to attack him from the flanks.

[1][20][21] MacDougall was appointed to the Royal and Military Order of Saint Ferdinand by Isabella and also received the British Legion Medal for his service in Spain.

[25] The regiment was embodied as part of the regular army on 25 January 1855, in response to the war and a resulting offer of service from MacDougall.

Initially serving at Liverpool, it saw service in the south of England around Dover and Portsmouth and also travelled to Cork and Kinsale in Ireland.

[30] MacDougall convened the meeting that began the movement, alongside Admiral Sir Charles Napier and Alfred Bate Richards, at St Martin's Hall in London on 16 April 1859.

[1] In response to the pressure from the meeting and an outcry from the press, the War Office announced the official formation of the new corps on 12 May.