Maas has a university degree in International Law. He worked for the Dutch Ministry for the Environment. As a Legal Counsel, he joined the Dutch delegation to numerous meetings of a wide variety of Multilateral Environmental Agreements, including the Montreal Protocol (Ozone), the Biosafety Protocol (GMOs), the UNFCCC (Climate Change), and the Rotterdam and Stockholm conventions (chemicals).
He (co-)chaired many UN negotiations, including the successful 2007 accelerated phase-out of HCFCs under the Montreal Protocol. Maas was head of the Dutch delegation to the UN climate talks under the UNFCCC. In addition, he has been Lead Negotiator of the EU to the UN climate negotiations.
He left the Dutch government in 2013 to become Executive Director of the Netherlands Committee for IUCN — the International Union for the Conservation of Nature. In early 2016, he joined DOB Ecology as its CEO.
Maas is married to Kirsten and they have three young children.