Construction on the East Block commenced by the end of 1859, at the same time as work on the Centre Block and Stent and Laver's West Block began. By the time it was completed in 1866, the building was four years behind schedule and costs had risen to $706,549, when $150,000 had originally been allocated. As the home of the office of the governor general and the offices for all the Cabinet ministers, the East Block was immediately occupied by the Viscount Monck and his prime minister, John A. Macdonald, who occupied the room at the south west corner of the second floor; the same room was also used by Lester B. Pearson when he was Secretary for External Affairs. George-Étienne Cartier used an office at the northern end of the west wing, which was thereafter used by every prime minister until Pierre Trudeau. The Queen's Privy Council chamber was used for cabinet meetings for 105 years, and was where the British North America Act 1867 was formulated, decisions about the Red River Rebellions were made, and Canada's war involvement was orchestrated. As the number of staff on Parliament Hill grew with the expansion of the country, however, ever more office space was desired; in 1910 a new wing was added to the rear of the East Block, enclosing the courtyard, providing area for the Civil Service and vault space, and costing $359,121.The 1910 wing had six massive vaults, originally used for storing the nation's financial treasures before the Bank of Canada was constructed. Rumours have circulated for years that gold was once kept here. The vaults have been converted to office space, but the original doors have been preserved.

However, restoration of some interiors began in 1966, seeing Macdonald's office and the privy council chamber returned to an 1870 appearance, and after which the public was allowed entry for a few hours each weekend. Another spurt of renovations were completed in 1981, wherein the governor general's former office was restored, and then the mechanical and electrical systems, and the masonry of the 1910 wing, which had never previously been worked on, were renovated in 1997. Rehabilitation work has also begun on the Northwest Tower, which will include structural and seismic reinforcement and the installation of a new copper roof.