The role covers BHP Group Limited and BHP Group Plc.
Cox has been with BHP since 2014 and previously was a partner at Herbert Smith Freehills for over a decade.
The company said Rachel Agnew remained a company secretary of both entities, while vice president governance Geof Stapledon would remain a company secretary of BHP Group Plc and take on the role at BHP Group Limited as well.
All changes will take effect on March 1.