Finance ministers from the world's biggest economies on Saturday failed to get the United States to renew an anti-protectionist pledge and a vow to fight climate change, in the face of Donald Trump's "America First" push.
Mnuchin has said trade needs to be "fair", which would be a step back from the group's previous blanket condemnation of trade barriers.
Chinese Finance Minister Xiao Jie said his country would "unswervingly oppose trade protectionism".
"But one of the European officials said this was not uncommon for a new administration, which was still formalizing its viewpoint on an array of complicated matters".
And to the extent that agreements are old agreements and need to be renegotiated we'll consider that as well, ' he said.
U.S. Treasury Secretary Steven Mnuchin sought to downplay the divisions, saying that the language opposing protectionism was "not really relevant" any longer.
The failure of the world's financial leaders to agree on resisting protectionism and support free trade marks a setback in the G20 process and poses a risk for growth of export-driven economies such as host Germany, economists said on Sunday.
U.S. President Donald Trump's administration says it wants to make global trade rules fairer, raising concerns among exporting nations like Germany.
The WTO operates a system of negotiated trade rules and serves as a forum for resolving disputes.
German Chancellor Angela Merkel and Chinese President Xi Jinping agreed to work together for free trade and open markets, a German government spokesman said.
"We believe in free trade, we are in one of the largest markets in the world, we are one of the largest trading partners in the world, trade has been good for us, it has been good for other people", Mnuchin said.
But trade deals that now exist are not always balanced, he said, warning that Washington will not hesitate to renegotiate them.
As well as the free trade agreement falling apart, the deal to prevent climate change was also undermined.
Finance ministers from 20 world powers have struggled at talks in Germany to find a common position on trade in the face of resistance from the United States.
Trump has called global warming a "hoax" concocted by China to hurt US industry and vowed to scrap the Paris climate accord aimed at curbing greenhouse gas emissions. This more casual meeting of finance ministers will be followed by a formal gathering of national leaders in Hamburg, Germany, in July. Representatives from the 19 countries and the European Union will meet for a formal summit in July.