Young Rainy was intended for his father's profession, but he was caught by the evangelical fervour of the Disruption movement, and after studying for the Free Church he became a minister, first in Aberdeenshire and then in Edinburgh, till in 1862 he was elected professor of Church history in the theological seminary, New College, a post he only resigned in 1900.
Among the public offices held by the earl were those of lordlieutenant of Aberdeenshire, president of the society of Antiquaries from 1812 to 1846 and fellow of the Royal Society.
By draining the land, by planting millions of trees and by erecting numerous buildings, he greatly improved the condition of his Aberdeenshire estates, and studied continually the welfare of his dependants.
In a single season Aberdeenshire suffered nearly 90,000 worth of damage owing to the ravages of the diamond back moth on the root crops; in New York state the codling moth caused a loss of $3,000,000 to apple-growers.
Leaving Edward, now his only brother in blood and almost his equal in arms, in Galloway, he suddenly transferred his own operations to Aberdeenshire.