Career
In style he was close to the Swedish painters of his generation known as 1909 års män (which translates as The Men of 1909) including Isaac Grünewald, Tor Bjurström, Arthur Percy, Birger Simonsson och Sigfrid Ullman. His production was quite small, roughly 50 works are known including works from art school and self-portraits. Two of his paintings are at Moderna museet in Stockholm.
Lönnberg was also a long-distance runner.
He was to compete marathon at the 1912 Summer Olympics but did not start the race or did not complete it due to illness. He came to Paris in January 1914, where he became a part of a circle of Swedish painters on Montparnasse including Isaac Grünewald, Einar Jolin, and Eric Detthow.
After the World War I broke out he volunteered for France, joining the French Foreign Legion. He died in service in 1918, the year the war ended.