John was born on the 14th of September 1872, in Kalmar, Sweden, and he emigrated to the United States through California, where he enlisted in the US Marine Corps in 1896. Though not much of his early and later life is recorded, it is known that he served in the Spanish-American War and also deployed to China in support of the Boxer Rebellion, where his actions as a Corporal would earn him the Medal of Honor. The citation reads:

In the presence of the enemy during the battle of Peking, China, 20 June to 16 July 1900, Dahlgren distinguished himself by meritorious conduct.

John was part of a guard team that also constructed barriers that were used to help protect the city from the invading rebels. John received the Medal of Honor on the 19th of July 1901 and was discharged at some point in 1901. John Olof Dahlgren died on the 11th of February 1963, at the age of 90 and he is buried with his wife Ada in the Golden Gate National Cemetery: Section Z, Grave 1950.