Tonko started his career as fixed income analyst in 1998. In 2000 he joined ABN AMRO Asset Management as an alternative fixed income portfolio manager, managing $250 million in institutional mandates.

In 2003 Tonko founded Dynamic Credit in New York. During the global financial crisis in 2008-2009 Tonko advised the Dutch Central Bank and the Ministry of Finance on the solution for the US mortgage issues ING Bank was facing on a $39 billion portfolio. This advice was subsequently examined by a Parliamentary Inquiry Commission and gave the taxpayer a positive return of €1.4 billion.

Currently Tonko is CEO of Dynamic Credit Group, which includes asset management, advisory, analytics and direct lending businesses, with offices in New York, Amsterdam and Jakarta.

Tonko studied Monetary Economics at the University of Groningen and continued his studies of Credit Risk Modeling at Stanford Business School in 2000.