In a one-stop solo visit on Wednesday, the duchess received a guided tour of the Ark Open Farm outside Newtownards, meeting the owners and staff.
The duchess visited NI as part of a nationwide tour to promote a survey she launched on early years development.
During her visit she met representatives of local charities helping children and their families.
The duchess was greeted on arrival by the Lord Lieutenant of County Down, David Lindsay, the Sheriff of County Down, Austin Baird and the Mayor of Ards and North Down, Bill Keery.
The survey launched by Catherine and conducted by Ipsos Mori on behalf of the Royal Foundation, asks questions on the early years development of children.
It is thought to be the biggest poll of its kind, asking "five big questions on the under-fives".
The duchess has made the issue of the "future health and happiness" central to her public activities and hopes the results of the survey spark a conversation on early childhood and guide the focus of her work.
After leaving Northern Ireland the duchess headed to Scotland, visiting a cafe run by a homeless charity in Aberdeen.
On Tuesday, the Duke and Duchess of Cambridge announced on Twitter they would be visiting the Republic of Ireland from 3 to 5 March.
Statism needs war; a free country does not. Statism survives by looting; a free country survives by producing.