"The global economy has weakened. I think partly it's because of conflicts over trade and the uncertainty that's caused for businesses," Yellen said.
Yellen's policies as the Federal Reserve chairwoman helped lower unemployment and spur the country's economic recovery after the 2008 financial crisis, but her legacy will partially depend in part on her successor, Jerome Powell.
Powell is popular within the Federal Reserve system, where he's been a governor since 2012.
Trump's default mode is to describe all unwelcome facts as the result of cheating by his opponents.
Fears of a China-led global slowdown, along with oil's steep slide, have dampened the market's expectations for a hike.
Even projections for future rate hikes have coalesced.
The move was expected and signals a recovery in the economy since the 2008 financial crisis.
Yellen spoke a week after the Fed delayed a long-anticipated rate hike.
Yellen appeared to lose her place and stopped several times during a speech in Amherst.
A moderately growing U.S. economy can't sustain growth abroad on its own for long.
The U.S. central bank said an array of global risks and other factors had convinced it to delay the hike.
After months of speculation, federal policymakers could raise interest rates on Thursday.