Before coming to be director of NYSE Euronext in 2002, Williamson was chairman of LIFFE twice, with stints in the position from 1985 to 1988 and from 1998 to 2003.  He brought profound change to the exchange, embracing new technology and abandoning open outcry for electronic trading, restructuring the whole business.
He has also held directorships at the Financial Services Authority, Templeton Emerging Markets Investment Trust, Resolution plc, Climate Exchange plc, and HSBC Holdings. As of June 2013, along with being the director at NYSE Euronext, he is the director of MT Fund Management Ltd, Politeia, and is a trustee of the Winston Churchill Memorial Trust. Williamson was inducted into the Futures Hall of Fame in 2007. 
Williamson is a graduate of Trinity College in Dublin, Ireland.