Details about his life are scarce. He was likely born in Antwerp. His father was a nobleman originally from Tholen who had moved to Antwerp where he was operating as a cloth merchant. His father was active in the local Chamber of rhetoric De Olyftack in Antwerp and was the chambers dean in 1552–3. The van Dalem family was a cultured family. Cornelis and his older brother Lodewijk likely enjoyed a humanistic education and were both trained as painters with the obscure artist Jan Adriaensens. Cornelis commenced his training in 1543 and became a master in the Guild of Saint Luke in 1556. The same year married Beatrix van Liedekercke, a member of a wealthy family.
Despite being a member of the Guild of Saint Luke, Cornelis van Dalem only practiced painting as an amateur and he remained a merchant his entire career. He was independently wealthy and did not need to rely on his art to make a living. However, he regarded art as an important part of his life as is demonstrated by the way he decorated the facade of his Antwerp residence. Cornelis had the façade of his house decorated with a splendour not seen in Northern Europe. The façade had a relief of the goddess Pictura in front of an easel, of Minerva and Mercury as well the stone busts of Durer and Jan van Eyck with laudatory inscriptions.