SYDNEY-- Rio Tinto PLC extended the tenure of chief executive Sam Walsh, rewarding him for restoring capital discipline after a run of hefty write-downs from soured acquisitions.

Rio Tinto also extended the term of finance chief, Chris Lynch. Together, the decisions eliminate a layer of uncertainty at Rio Tinto after the company recently became a takeover target of Glencore PLC.

Mr. Walsh, who succeeded former chief executive Tom Albanese in January 2013, faced the task of steering Rio Tinto through a period of weakening resources prices and slowing demand, as a decadelong mining commodity-price boom came to a halt. He was quick to make dramatic changes, stripping billions of dollars of costs out of the business and cutting jobs to safeguard profits.

"Sam has made no secret of the fact that he loves his job and would like to continue well beyond next year," chairman Jan du Plessis said Thursday. "Given his performance and his enthusiasm to continue in the role, the decision to extend his tenure has been an easy one for the board."

The Anglo-Australian company said Mr. Walsh and Mr. Lynch would be moved to open-ended contracts after the annual general meetings in Australia and the U.K. next year. The contracts were slated to end Dec. 31, 2015 and Feb. 28, 2017, respectively.

Mr. du Plessis said the move to rolling contracts represented "long-term, open-ended commitments to the company."

The leadership decisions also come amid speculation about the future of London-based Rio Tinto.

The world's No. 2 iron-ore miner rebuffed Glencore's approach in July about a merger, saying a deal wasn't in the best interests of shareholders. The company said earlier this month it hadn't been in contact with the commodities producer and trader on the matter since early August.

Mr. Walsh and his team argue they are best placed to strengthen the company's footing and bolster returns to shareholders. Rio Tinto more than doubled its profit in the first half of 2014, the company said in August, as it signaled plans to give back more cash to its investors starting next year.

Mr. Walsh's austerity drive marks a bold break with the company's past, one dotted with several big acquisitions that subsequently were written down when commodity prices slumped. His predecessor, Mr. Albanese, stepped aside after Rio wrote down more than $14 billion in assets, stemming from big aluminum and coal asset purchases made at the top of the market. The company has continued to face write-downs from acquisitions made before Mr. Walsh took the helm.

Even with Mr. Walsh's bold measures, Rio Tinto still faces challenges from a 40% slide in iron-ore prices this year, which some analysts say may hinder the company's ability to buy back shares as expected. Rio Tinto relies on iron ore for the majority of its earnings.

Write to Rhiannon Hoyle at rhiannon.hoyle@wsj.com

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