Architect and designer Johannes Magdahl Nielsen was born on February 5, 1862 in Raerup, Denmark. The son of distillary owner Morten Nielsen and Elise Jacobsen, he first went into the mill building trade once he'd completed secondary schooling. After three years he entered the Odense Technical School, where he studied architecture and draftsmanship from 1879 to 1883. He then enrolled at the Royal Danish Academy of Fine Arts, Charlottenborg, and the School of Architecture, graduating in 1892. He received a scholarship to travel to Italy from 1893 - '85, where he would return in 1912 and 1920, studying ancient architecture.

Upon graduation Nielsen was employed by the Hector Estrup architectural firm. A prolific designer, he not only built dozens of official government buildings and private homes, he also found success as a restorer of medieval structures such as the Kronborg Castle, Ebeltoft Old Town Hall, and the Church of St. Nikolai, among others. He was elected a member of the Academy of Fine Arts' plenary assembly in 1904, the Academy Council from 1914 - 1931, and was employed by the city of Copenhagen as the inspector of royal buildings and structures. Among his most well known designs is the Safnahusio, also known as "The Culture House", in Reykjavic, Iceland, which is currently in use today as a museum and exhibition space.

In 1909 he was awarded the Eckersberg Medal from the Academy and a medal from the Association for the Beautification of the Capital in 1923. He was also given the Medal of Merit by Copenhagen and was appointed a Knight of Dannebrog, Dannebrogmand, and a Commander of the 2nd degress of Dannebrog.

Nielsen died in Hellerup, Denmark, on August 25, 1941.