William James Pirrie was born in Quebec, Canada on 31 May 1847 the only son of James Alexander Pirrie his wife Eliza (neé Montgomery). Pirrie's family lived at Conlig, County Down.
Following an education at the Royal Belfast Academical Institution, in 1862, he entered the firm of Harland and Wolff. So meteoric was his rise that by 1874 he had been made a partner. He would remain with the firm all his life. Pirrie nurtured a close and lucrartive working relationship with first Thomas Henry Ismay and later his son, Joseph Bruce Ismay who led the White Star Line. Harland and Wolff was the sole constructor for White Star, building its ships to the highest specifications on what was termed a "cost-plus" basis.
Pirrie was instrumental in the formation of the International Mercantile Marine under John Pierpont Morgan which consolidated maritime transport in the North Atlantic with obvious benefits to his firm.
He was Lord Mayor of Belfast from 1896 to 1897 and was made Baron Pirrie of Belfast in 1906 (Viscount Pirrie in 1921 when George V visited Belfast).
Lord Pirrie died at sea on 6 June 1924 while on a business tour of South America.