He was possibly a relation of David Decker, who was registered in the Haarlem guild as a pupil of Gerrit Claesz Bleker in Haarlem in 1640.
David Decker's only known artwork was formerly attributed to Cornelis and shows strong similarity with his work.
[1] Cornelis Gerritsz Decker became a member of the Haarlem Guild of St. Luke in 1643.
[2] He painted landscapes in the manner of Jacob van Ruisdael.
Some of the staffage in his landscapes was painted by Adriaen van Ostade, Philips Wouwerman and Johannes Lingelbach.