But before that discussion got underway, Trump expressed her thoughts about the tragic church shooting in Texas.
"God bless the people of Sutherland Springs, TX," she tweeted. "Our country's hearts are breaking for the victims & their families. We love & are with you!"
Trump, who serves as an advisor to her father President Donald Trump, and Mnuchin discussed the administration's tax plan before a crowd of more than 300 people.
Trump said she's particularly concerned about working families.
"Tax relief is about creating a culture of work and dignity in this country," she said, "and giving our working parents greater economic stability, so they can make the best choices for their own families."
Mnuchin said the administration will reduce the business tax rate and overhaul the tax code for the first time since Reagan was in office.
"Great to have joined @IvankaTrump @Reagan_Library to discuss #TaxReform," he tweeted afterward. "We'll cut taxes for families & make US businesses more competitive."
Mnuchin will discuss tax reform with Southern California business leaders.