The certification, a global sustainability standard for events used by Locog during its planning for the 2012 Olympics, launched in June.
The standard, developed following the success of event sustainability management system British Standard BS 8901, comprises a framework for events that helps planners reduce costs, carbon emissions and waste.
First Protocol managing director Mark Riches said: "We are delighted to be among the first to be certified with ISO20121. Delivering innovative and sustainable events that make a difference for our clients has always been a priority, but now more than ever we need to consider the impact of those events on the environment and the communities they touch.
"We are encouraging all our partners and suppliers to engage with the certification process so that collectively we can make a bigger impact sooner rather than later."