Marnix mainly advises, litigates, publishes and lectures in the field of procurement law.
More broadly speaking, he focuses on issues relating to the economic activities of public authorities (like state aid and the Public Enterprises (Market Activities) Act). Marnix also has extensive experience in gambling laws and regulations.

His clients commend his analytical skills and his “velvet pen”. Marnix views language as a lawyer’s most important weapon. A good lawyer uses words like a sword, but often also like a shield for his clients, not only on paper, but also in court.

Before becoming a lawyer, Marnix worked for several ministries and regulatory authorities.
Besides being a lawyer, Marnix also studied economics and is therefore better able to translate the legal aspects into the economic world in which his clients, often public/semi-public authorities, operate.