In 1862 Pirrie became apprentice to the Belfast shipbuilding firm of Harland and Wolff. By the time he was 27 he had been made a partner and was soon left in almost exclusive control. He traveled widely to gain experience in ship design and to study practical shipping requirements. He contributed much to the burgeoning steel shipbuilding industry, and for many years the largest passenger liners in the world came from his yards, notably the Olympic, the Britannic, and the Titanic. Pirrie was also prominent in the development of the diesel engine for marine propulsion.

Created a baron in 1906, Pirrie became a viscount in 1921. As comptroller general of merchant shipbuilding in 1918, he helped replace British shipping lost to submarine warfare. He was also mainly responsible for introducing the idea of standardizing ships, a principle that was adopted in Britain and the United States during World War II.

Pirrie's marriage was childless, and the peerage became extinct at his death.