Magnus Poulsson is most famous for designing the Oslo City Hall, together with Arnstein Arneberg. He is also known for his work on Bærum City Hall (Bærum Rådhus) in Sandvika (1925 and 1958), KNA Hotel in Oslo (1931), Eystein church in Hjerkin and Haslum Chapel in Bærum. Poulsson's work also included private residences, office buildings, churches and interiors. Magnus Poulsson was particularly known for recreating the tradition of Norwegian folk wooden architecture.
Poulsson was chairman of the Ancient Monuments Society 1917-1930 and chairman of the supervisory committee for the Nidaros Cathedral reconstruction from 1931–58. He was an honorary member of the Danish AKAD and a member of the Swedish Royal Academies for Fria Art Erna.