Castro is a sexologist and director of the Cuban National Center for Sex Education and has long been an advocate for LGBT equality in Cuba, the Philadelphia Inquirer said.
She pledged to advocate for gay marriage during her speech at the Equality Forum conference Saturday afternoon.
Castro also defended her country's government, which has come under fire for repressing political dissent, the Inquirer said.
"There is not any government or any country who has the right to impose or make decisions to the other ones regarding the human-rights area," she said.
The U.S. State Department initially denied Castro clearance to travel to Philadelphia, but later reversed the decision.
But some lawmakers did criticize Castro's visit.
"For a person like Mariela Castro to attend a conference on civil rights for lesbians, gays, bisexual and transgender people, and to receive an award, is shameful, pathetic and a ruse," Rep. Ileana Ros-Lehtinen, R-Fla., said. "The words equality and human rights don't exist in the vocabulary of the Castro tyranny."