His father, Julius James Sundby (1837–1911) was a farmer and a political spokesman for Høyre, the conservatives, in Vestby, married to Othilie Strand (1841–1909).

He was initially unwilling to become a council of the state, but gave in due to an ultimatum from Peder Kolstad when he was the Minister of Agriculture in 1931.

He was also in trusted positions in Norske Melkeprodusenters Landsforbund and Meieribrukets Sentralstyre, as well as being the chairman of Felleskontoret for the central federation of agriculture 1945-57.

Sundby was a clever academic- and political writer, and he was the author behind many books questioning and discussing agricultural concerns.

Jon Sundbys medal of honor is awarded every second year to a person who has conducted achievements within dairy or agriculture more over.