He received his Ph.D. in Biology (cum laude) from the University of Amsterdam in 1986. From 1986-1991 he obtained his PhD on the effects of trace metals on Chironomid larvae (University of Amsterdam). In 1991 he defended his thesis entitled "Trace metal ecotoxicokinetics of chironomids.
Since 1991, Klaas Timmermans has been working as a (senior) scientist at the Royal Netherlands Institute for Sea Research (NIOZ), first within the Department of Marine Chemistry and Geology, from 2008 to 2016 at the Department of Biological Oceanography and from 2016 as Head of Department at the Department of Estuarine and Delta Systems at NIOZ in Yerseke.
From September 1, 2014, Klaas Timmermans was appointed Honorary Professor of Marine Plant Biomass in the Department of Ocean Ecosystems in ESRIG: Energy and Sustainability Research Institute Groningen at the University of Groningen (Faculty of Science and Engineering).
Since June 2019, he is lecturer Aquaculture in Delta Areas at HZ University of Applied Sciences.