Abdulaziz al-Saghyir quit due to health reasons, but will remain on Mobily's board, the company said in a statement to Riyadh's bourse.
Mobily has appointed Suliman bin Abdulrahman al-Gwaiz as chairman, effective February 23.
Gwaiz is also chairman of Banque Saudi Fransi and governor of state-owned fund the General Organization for Social Insurance, the statement said.
The telecom operator reported a surprise fourth-quarter loss of 2.28 billion riyals ($607.9 million), while in November it also cut its profits for 2013 and the first half of 2014 by a combined 1.43 billion riyals.