Cockerill began work in Verviers, near Liège, in 1799. His work involved manufacturing machines for the spinning and carding of wool for the firm Simonis et Biolley, run by Iwan Simonis and his sister Marie Anne Simonis. He then brought his family from England and settled in Belgium. The region was part of the French Republic during the French Revolutionary Wars, but this did not affect Cockerill's economic fortunes. He was joined in 1802 by James Holden who initially worked for him as an assistant and later set up his own business.