Hagemann went to school in Aarhus before studying Applied Sciences at the College of Advanced Technology in Copenhagen from 1860 to 1865.

In 1865, when he had completed his exams, Kryolit Mine og Handelsselskabet sent him to the US to oversee the deliveries of cryolite to the Pennsylvania Salt Manufacturing Company.

With inspiration from David Alter's nearby production site, Hagemann began to work on improving methods to manufacture and purify bromine from salt wells.

During a visit to Europe in the summer of 1869, C. F. Tietgen convinced Hagemann to purchase the Øresund cryolite factory in a partnership with Vilhelm Jørgensen (1844–1925).

[1] Hagemann and Vilhelm Jørgensen also constructed Hinnerup Natroncellulosefabrik for Count Mogens Frijs and operated it on a ten-year lease.

Hagemann, who was appointed to chief technical officer in 1882, was also responsible for planning the new sugar manufacturing plants in Nakskov (1881), Assens (1883) and Stege (1883).

In 1894, Hagemann purchased the sugar plantation La Grange on Saint Croix and immediately began to modernize its operations.

Hagemann retired from the position as chief technical officer of De Danske Sukkerfabrikker in 1897 but then served as chairman of the company from 1898 until his death.

She was the daughter of local merchant, savings bank manager and deputy consul Bertel Brun (1820–1894) and Margrethe Sophie Frederikke Brin née Brasch (1825–1893).