Frenzel was born in Zehdenick, Templin district on 20 August 1911. His father had worked for the railroad and was a local official of the Social Democratic Party of Germany. Karl completed primary school from 1918 to 1926 in Oranienburg and then apprenticed as a carpenter. During this time, he was a member of the socialist carpenter's union. However, after passing the qualifying carpentry exam in 1930 he found himself unemployed. Later he found work for a short time as a butcher.