The teenager travelled to the global body's headquarters in Switzerland to address the body as part of World Down Syndrome Day, this week, and the respective 2017 campaign #MyVoiceMyCommunity.
Humberstone told the UN that the disorder has in no way held her back. In fact, it's given her opportunities. She said she's posed for the Radical Beauty Project earlier in the year, a fashion campaign that challenged industry norms by only using models with Down Syndrome, and plans to live away from her parents after finishing university.
"After college, I am planning to live independently just like everybody else." Humberstone said.
Humberstone's UN visit coincides with the release of a new test to more accurately determine if a pregnant woman is carrying a baby with Down Syndrome. The teen used her speech to challenge the efficacy of the examination.
"I love everybody... I hope everybody loves me," she said.
"I have Down Syndrome - is that so wrong? So why all this testing? Why? I love my life!"
Kathleen's mum Denise discussed the pride she felt in having her daughter speak in a room full of such influential policy makers while sharing Kathleen's view on the new test.
"I feel rather mixed emotions right now; on the one hand I'm incredibly proud of Kathleen's achievement... [but] whatever happened to unconditional love?" Denise said to the BBC.
One thing is for certain, the Humberstones have a lot of supporters with a large number of people getting behind World Down Syndrome Day, as they shared their stories on social media.