Sigurd Frosterus was born on 4 July 1876 in Asikkala, Päijänne Tavastia region. Frosterus studied architecture at the Polytechnical Institute of Helsinki and received his diploma in 1902. He was married to Emmy Frosterus (nee von Kraemer) and had two daughters.
The years 1902–1904 Forsterus had a shared office with architect Gustaf Strengell. Together they designed the Art Nouveau style private residence, Tamminiemi, which was completed in 1904. It was donated to the Finnish State in 1940 and has been the Urho Kekkonen museum since the year 1987.
Frosterus was an active debater, an art theorist and critic, an art collector and an architecture journalist. He was even the editor in chief of the Finnish Architectural Review the years 1908–1911. Frosterus’ main design works are the Stockmann Department Store in Helsinki and the Vanaja Mansion in Hämeenlinna. He designed summer villa for his family, Villa Frosterus, which was built in 1913 to the Espoo archipelago.
Sigurd Frosterus died on 2 March 1956 in Helsinki. He willed his art collection to the Amos Andersson Art Museum (now Amos Rex Art Museum).