At the 1734 British general election, he was returned by his brother-in-law, William Duff, later Lord Braco, as Member of Parliament for Banffshire.

At the 1747 British general election, he was returned unopposed as MP for Banff as an Old Whig, but stood down in 1754 in favour of Lord Braco's son, now of age.

[4] With the outbreak of the Seven Years' War in 1756, Abercrombie was promoted major general and ordered to America as second in command to Lord Loudoun for the upcoming campaigns against the French.

[1] Abercrombie commanded a brigade at Louisbourg in 1757 and became Commander-in-Chief of the British forces in North America after Loudoun's departure in December.

He managed the remarkable feat of assembling fifteen thousand troops at Fort Edward and moving them and their supplies through the wilderness.