Tan Sri Ahmad Nizam Salleh has held various positions such as analyst, planner and project coordinator in corporate planning and finance sectors in PETRONAS' Corporate Head Office from 1981 to 1987. He was also a member of the PGU I and II project management team and project directorate for the LNG Dua project. From 1988 to 2002, Tan Sri Ahmad Nizam Salleh held various senior positions in PETRONAS' Holding Company, including as Head of Crude Oil Group and Head of Group Treasury. In 2004, he assumed the position of Managing Director/Chief Executive Officer of Malaysia LNG Group of Companies and was promoted to Vice President, Corporate Services Division of PETRONAS in 2007.

Tan Sri Ahmad Nizam Salleh was a member of the PETRONAS Management Committee from 2004 to 2010. From 2007 to 2010 he served as the Chairman of PETRONAS Central Tender Committee and Chairman of PETRONAS Risk Management Council. He also represented PETRONAS as a Board Member of MISC Berhad, Putrajaya Holdings Berhad and Bintulu Port Holdings Berhad, and served in several boards of PETRONAS' subsidiary companies.

From July 2010 to August 2015, Tan Sri Ahmad Nizam Salleh was the Managing Director/Chief Executive Officer of Engen Ltd, South Africa, operating in 20 countries in Southern Africa and Indian Ocean Islands.

Tan Sri Ahmad Nizam Salleh is currently the Pro-Chancellor of Universiti Teknologi PETRONAS and a member of the Board of Directors of the University of Malaysia Terengganu.

Tan Sri Ahmad Nizam Salleh was appointed as Chairman of PETRONAS effective 1 August 2018.