Nannfeldt studied natural history at the University of Uppsala and obtained a doctorate degree in 1932.

[5] He also treated taxonomy and biogeography of various groups of vascular plants, e.g. the arctic Poa laxa complex.

[6] Nannfeldt published the exsiccata work Fungi Exsiccati Suecici, praesertim Upsalienses together with Lennart Holm and others.

The fungal genus Nannfeldtiella was named in his honour by Franz Petrak in 1947, (now listed as a synonym of Pseudombrophila aggregata (Eckblad) Harmaja.

[8]) then in 1960, mycologist Richard Korf circumscribed Jafnea, which is a genus of fungi in the family Pyronemataceae.