In 1891 he obtained his PhD from the University of Uppsala, spending the next twelve years as an inspector in the fisheries service.

In regard to his zoological research, his primary focus dealt with mammals, birds and fish, but he also made significant contributions in his studies of reptiles and amphibians.

He was the binomial authority of numerous taxa, and has several species named after him, such as Onykia loennbergii (Japanese hooked squid)[1] and a subspecies of the Brown Skua.

[2] The Belgian-British zoologist George Albert Boulenger named a small, secretive, venomous, endemic New Guinea elapid snake Apisthocalamus loennbergii in his honour, although this species is now synonymised with Toxicocalamus loriae (Loria's forest snake).

[3] In his obituary in the ornithological journal, Ibis, it was written: "that since the days of Linnaeus hardly anyone has known so much about so many branches of zoology as Lönnberg".