He apprenticed as a merchant in Holbæk from 1867 and later studied at Grüner's business academy before working for several large enterprises.

In 1922, shortly before his death, he acquired an extensive coin collection which had belonged to the counts of Brahesminde.

The British part of his collection was sold in auction in London after his death, except for the coins from Northumberland, East Anglia and Ireland, which were donated to the Royal Danish Coin and Medal Collection at the National Museum of Denmark.

[2] According to Bruun's will, his collection of coins, bank notes and medals from Denmark, Norway, Sweden, Schlesvig and Holstein, as well as his numismatic library, was to act as a back-up for the Royal Danish Coin and Medal Collection for a period of one hundred years,[2] before being sold for the benefit of his descendants.

[3] It was initially deposited at the Museum of National History at Frederiksborg Castle, but was transferred to Nationalbanken (the central bank of Denmark) in Copenhagen.