She added: "What we have to do is get out of the ballrooms, out of Stormont and into the communities where people live, where they do not have that lasting hope of optimism."
This was Mrs Clinton's seventh visit to the North.
She and her husband, former US president Bill Clinton, were key figures in helping to carve out the peace process during the 1990s.
It comes amid fresh violence in Belfast, with attacks on the Alliance Party and the discovery of two bombs in Derry and Co Down.
Mrs Clinton said she wanted to continue to work with political leaders after she stands down from politics next year.
She said: "There is a lot to be proud of, but I want to offer a cautionary word because if we do not focus on a community level, on that interface, we will not have really achieved the peace that has been worked for.
"I want to remain involved as a friend, advocate and cheerleader for what we already achieved.
"Let us reach out to those who do not yet feel in their heart what has been achieved."