Hannes Meyer departs as CFO after 11 years of service but will remain with the company, First Quantum said.
MacWilliam has been with the company since 2019, leading business development and investor relations.
He previously served as CFO of Canadian base metals company Nevsun Resources until it was acquired by China's Zijin Mining.
He was a founding member of mining equity fund, X2 Resources, and held finance and corporate development roles at Anglo American in copper, iron ore, and platinum divisions.
Badenhorst joined First Quantum in 1996 for the design, construction, and commissioning of the Bwana Mkubwa tailings project in Zambia.
Since 2004, he has held general manager positions at several of the company's operations.
He was promoted to operations director in 2019.
"He is very well placed to provide significant impetus to our production and optimization efforts in the current challenging environment," CEO Tristan Pascall said.
"I want to thank Hannes for his leadership, commitment, and support.
"He has stewarded the Company's finances through challenging times and it is a reflection of his efforts that he leaves the balance sheet in a much improved position," he said.
First Quantum's share price was down 10% day on day at C$21.01 (US$15.97) on 1 September. The company has a market capitalisation of C$14.53 billion.