James Abercrombie combined army service with parliamentary patronage before commanding British forces in North America. Elected for Banffshire in 1734, he became lieutenant governor of Stirling Castle in 1739. The wars of the 1740s brought promotion: he served as quartermaster general in the unsuccessful expedition against Lorient in 1746 and was wounded at Hulst the following year. After a period away from active campaigning, he returned to war during the struggle for Canada.

Sent to North America in 1756 as deputy to the Earl of Loudoun, he became a major general and colonel of the 44th Foot. He joined the force assembled at Halifax for the abandoned Louisbourg expedition of 1757. When Loudoun was recalled, Abercrombie became commander in chief. In 1758 he personally led the army assigned to advance by Lake George toward the French fort of Carillon, later Ticonderoga, while separate British armies operated against Louisbourg and Fort Duquesne.

His large force landed near Carillon in July. The death of Lord Howe during the advance deprived it of an energetic commander. On July 8 Abercrombie ordered repeated attacks against French defenses without first bringing up sufficient artillery. The assaults failed with heavy losses, and he withdrew to Lake George. John Bradstreet's subsequent success at Fort Frontenac partly offset the campaign's failure. Jeffery Amherst replaced Abercrombie, who returned to Britain and spent his later years largely in retirement at Glassaugh.