Colin was the 6th of 11 children of Ebenezer Robertson Cardno (1845-1910) & Jane Webster (1852-192). He married Frances Buchanan Macindoe (1886-1916) in Auckland on 10 Aug 1910 & had a son, Colin Grierson Cardno (1911-1958).
He next married Helena Jessie Simmons in Auckland in 1918