The baby is the first daughter for the princess and husband Jack Brooksbank.
The King toured an after-school, urban farming effort that works with young people affected by food insecurity.
William and Kate were married on April 29, 2011, at Westminster Abbey.
The royal family's visit to New York City comes in the wake of the shooting at the White House Correspondents' Dinner, which has prompted officials to take a second look at security.
King Charles III acknowledged "times of great uncertainty" as he expressed gratitude to the American people and marked the 250th anniversary of independence from Britain in a speech to the U.S. Congress that highlighted the bonds between the two countries at a time of political turmoil.
Britain's royal family is leading commemorations for what would be the 100th birthday of the late Queen Elizabeth II.