"Her social activism, her passion for justice and for the cause of the oppressed, were inspired by the Gospel, her faith, and the example of the saints," Pope Francis said during his speech of the Catholic convert who died in 1980 at age 83.
He praised her "tireless work" and called her a "Servant of God."
Thomas Merton was a writer and theologian, having written more than 70 books, many of which were devoted to religious thought.
"He remains a source of spiritual inspiration and a guide for many people," Pope Francis said of the monk who died in 1968 at age 53.
The pontiff went on to quote directly from Merton's autobiography.
"Merton was above all a man of prayer, a thinker who challenged the certitudes of his time and opened new horizons for souls and for the Church. He was also a man of dialogue, a promoter of peace between peoples and religions," the pope said, reiterating one of the major themes of his U.S. trip this week.