Mas Achmad Santosa (S.H., LL.M.) started his carrier serving as a public interest lawyer at the Legal Aid Jakarta under the mentorship of the late Prof. Dr. Adnan Buyung Nasution (1980-1987). After returning from Canada and obtained his Master of Laws degree (LL.M) from the Osgoode Hall Law School of York University, Toronto, Santosa continued his tenure as a public interest lawyer at the Legal Aid Foundation of Indonesia (YLBHI) from 1990 to 1993. In 1993, Santosa established the Indonesian Centre for Environmental Law (ICEL) and served as the Executive Director for the organization. He was also active as the Chief of National Presidium for Friends of the Earth Indonesia or famously known domestically as WALHI (Wahana Lingkungan Hidup Indonesia), an Indonesian non-profit organization for environmental issues. During the period of 2010-2013, Santosa assisted the Government of Indonesia by joining the REDD+ Task Force, serving as the Head of Working Group on Legal Review and Law Enforcement.
Presently, he is assisting the Minister for Marine Affairs and Fisheries of the Republic of Indonesia as the Head of the Task Force on the Prevention and Eradication of Illegal, Unreported and Unregulated (IUU) Fishing (2014-2015). He is now active as the Coordinator of Special Advisors for the Presidential Task Force to Combat Illegal Fishing, established by President Joko Widodo and Commandeered by the Minister of Marine Affairs and Fisheries of the Republic of Indonesia.
Santosa is active as a legal and judicial activist and is also a public figure for anti-corruption movement in Indonesia.