Hallam succeeds Peter Cash, who is stepping down as CEO after four-and-half years in the job.
A circa-A$100 million company currently producing 80,000-90,000ozpa of gold at Nullagine in Western Australia's north, Millennium is looking to grow its production profile through development of new underground and larger-scale openpit mines.
As well as Metals X, Hallam had a length stint early in his career with Western Mining Corporation where he led strategic and business planning. He was part of the senior leadership team at Metals X between 2004 and 2018.
"Warren is a highly accomplished and well-regarded mining executive who is well-known for his many achievements during a distinguished career at Metals X and, earlier in his career, at Western Mining Corporation," Millennium chairman Greg Bittar said.
"His extensive technical knowledge, hands-on operating experience and financial and commercial acumen will be invaluable as we work to complete the various expansion projects currently underway at Nullagine. At the same time Warren will lead a strategic review designed to optimise our operating strategy, reduce costs and ensure we are well positioned to take advantage of the current strong gold price environment and unlock the full value of the Nullagine project for our shareholders."