He served as Commander-in-Chief, India between 1770 and 1773. Barker went to India in 1749 and in 1757, during the Seven Years" War, commanded the artillery at the Capture of Chandannagar and at the Battle of Plassey. In 1762 he went on an expedition to Manila in the Philippines.
He was knighted on 16 January 1764.
Two years later he returned to India to protect the Nawab wazir of Oudh Shuja-ud-Daula. In 1769 he became Commander-in-Chief, India but exceeded his authority by committing the East India Company to guaranteeing a treaty and by confronting a possible Maratha Empire invasion.
He was elected a Fellow of the Royal Society in 1775. In 1781 he was created a baronet, of Bushbridge in the County of Surrey.
They lived at Bushbridge near Godalming in Surrey.