He was born in Kristiania,[1] the eldest son of Ellef Ringnes (1842–1929), brewery owner and benefactor, and his wife Kaja, née Maartmann (1851–1933).
He was a nephew of Amund Ringnes and maternal grandson of Knud Geelmuyden Fleischer Maartmann, his namesake.
[2] He graduated from Kristiania Technical School in 1896, and studied in England, the United States and at the University of Bonn from 1898 to 1899.
His uncle on the maternal side, Harald Sigvart Maartmann, took over the company in 1901,[2] and Knud Ringnes became chief executive officer in 1921.
He was a supervisory council member of Folketeatret (chair), Nationaltheatret, Forsikringsaktieselskabet Norden, Norske Liv and Christiania Bank og Kreditkasse.