[2] Sempill was made lieutenant-colonel of the 19th Regiment of Foot on 12 July 1731, and succeeded the Earl of Crawford as colonel of the Black Watch on 14 January 1741.
He received allowances to enable the officers belonging to the corps to provide themselves with baggage horses upon their going to Flanders.
[4] He commanded in the town of Aeth when it was besieged by the French, and that regiment made a gallant defence.
[2] Appointed colonel of the 25th Regiment of Foot on 9 April 1745, Sempill was promoted brigadier-general 9 June 1745.
In the middle of August following he arrived at Aberdeen and assumed the command of the troops stationed in that quarter.