Born in Saxony, Mummert volunteered for the army of Imperial Germany in 1914, upon the outbreak of the First World War. He was commissioned a reserve leutnant (lieutenant) two years later[1] and was awarded the Iron Cross, 2nd Class, a Prussian decoration.[2] He left the army after the end of the war but in 1936 he joined the Heer (Army) branch of the Wehrmacht.