The building was put up for an architectural competition in 1934. The competition was won by the young architects Jorma Järvi and Erik Lindroos. As the design work began, the team also came to include veteran architect Kaarlo Borg. In their designs, the architects divided all of the building's premises between the postal and telegraph operations, which had an effect on both the building's floor plans and the mutual arrangement of the premises. In the spirit of functionalism, the architects also wanted to emphasise the building's technical nature, speed and functionality. Likewise, they wanted to clad the building an entirely new material, a yellow postmark-patterned clinker made by Kupittaan Savi. The enduring material still adorns the building's stylish external surface. The building was completed in 1938.
Ever since its completion, Postitalo has played a significant role in the city and the life of its residents. The most renowned designers of their era – including Paavo Tynell, Runar Engblom, Arttu Brummer and Eino Kauria – had a say in the building's interiors, by making bespoke furniture and lighting fixtures for the premises. The open service counter of the postal hall represented a completely new approach to premises planning, and allowed customers to run their errands with the postal clerk without the traditional shutter in between.
In the 1950s, the building's space requirements grew and it was extended with additional floors. At the same time, the roof was equipped with the first helicopter field in Helsinki. The field was used during the Olympic Games held in the city in 1952. Of great architectural and cultural-historical significance, Postitalo is no longer open to repair work that could damage its value or style, and its postal hall, for instance, has been given the status of a listed site.
In 1998, the former Posti- ja Telelaitos split into two separate companies, changing the operations in the building. Today, the building is owned by the insurance company Ilmarinen and houses, among other things, plenty of office premises, a big convenience store, Helsinki City Library No. 10 and the restaurant Pääposti, open to all friends of good food, both an elegant lunch restaurant and a unique banquet premises.