Born Johannesburg 1949 and named Arnold Michael, his parents were political activists (mother a lawyer, father a trade unionist) and later exiles in Swaziland (from May 1962); schooled in Johannesburg and Swaziland (Waterford Kamhlaba), journalist, researcher-activist (in the UK), took a civil engineering degree at City University London (1974-1979); became an international 'cooperante' (solidarity worker) in Mozambique (1979-1988), and later a development specialist and policy analyst at DBSA (1988-1994). He was seconded to the Department of Water Affairs and Forestry in 1994 and later was appointed Deputy DG and then Director General (1997-2005). Muller was also a member of the 1st National Planning Commission, 2010-2015. After his contract with the DWAF was not renewed (2015) he joined the University of the Witwatersrand (Wits) School of Public & Development Management (P&DM), later renamed the Wits School of Governance (WSG), as an Adjunct Professor and became something of an academic, writing numerous papers for for peer reviewed journals and book chapters. He also continued his involvement with international organisations such as the United Nations, the Global Water Partnership, the ADB and SADC, as well as advising the SA government. He has always been, and continues to be, a prolific writer of articles in the South African national press.