 Janson's early years were spent at Fortis Green, and he collected beetles there and at Finchley, Hampstead and Highgate.In 1906 he made an expedition to Iceland adding to the collections of the National Museum of Iceland
During the First World War he made three trips to Ireland, particularly Co Kerry.Janson was a world authority on Cetoniinae publishing new genera and species mostly in "Cistula Entomologica" published by Janson and Co.
His world collection was purchased by Titus Valck Lucassen.