Carl Theodor Höijer was born on 20 February 1843 in Helsinki. Höijer’s father was an interior constructor Henrik Höijer, who wanted his son to become an architect.
Höijer studied at the Technical General School of Helsinki 1856–1860 (later Polytechnical Institute of Helsinki) to become an architect. Still, without taking matriculation examination, he couldn’t work at the Intendent’s Office, which was at the time the only place to work as an architect in Helsinki.
The years 1860–1863 Höijer worked as an assistant of G.T. Chiewitz, the regional architect of Turku and Pori province. He continued his studies at the Royal Swedish Academy of Fine Arts 1863–1868. After studies, he worked at the family business before working as an assistant of architect Hampus Dahlström, the head of the National Board of the Public Building 1870–1872.
Theodor Höijer’s generation established the first private architecture offices to Finland. Höijer established a shared office with architects Frans Sjöström, Axel Loenbohm and Theodor Decker in 1872 and a private office in 1876. Höijer became one of the most prolific Finnish Neo-Renaissance architects. He was a founding member of the Technical Society of Finland and a member of the Helsinki City Council 1881–1892. In 1895 he became the chair of the Architect Club of the Technical Society of Finland.
Höijer died at the age of 67, on 31 October 1910 in Helsinki.