In 1788 Van der Kemp arrived in New York. He bought a farm in Ulster County, New York in 1789, not far from Kingston and from 1777 the capital of New York. He had a house, a shed, a chicken coop, and a few slaves". In 1795, he was the founder of the Agricultural Society for the Western District of New York. Van der Kemp was linked to and visited in Kempwick by various Founding Fathers: John Adams, with whom he had an extensive and fascinating correspondence, Thomas Jefferson, George Washington, Benjamin Franklin and Alexander Hamilton. Mrs DeWitt Clinton, Hamilton and Tappan were all able to speak Dutch with his wife. He shared George Washington's interest in scientific agriculture after a visit to Mount Vernon. Van der Kemp withdrew from political life and threw himself into a new project at 'Nieuw Rotterdam', now called Constantia, in Oswego County.
Constantia had a fantastic location and a fine nature, and there was sufficient game for hunting and fish in the lake. Also Marc Isambard Brunel lived in the neighborhood. However, the bears and Native Americans were never far away, and his friend Adam Gerard Mappa lived at a day's travel away. Van der Kemp moved his family to Barneveld, now called Trenton. His house in Constantia was converted into a school. The Holland Land Company, that had taken into its service Gerrit Boon from Rotterdam, had meanwhile set up a proper village, planted ahorns and had a sugar mill built.
On the national scene, Van der Kemp's opinions influenced many of the great leaders of America. John Adams credited him with a "vast view of civilization" and respected his advice. His warm friendship with John and Abigail Adams endured till their deaths, and John Quincy Adams continued the relationship.
He persuaded Thomas Jefferson to anonymously publish his religious work a "Syllabus of an estimate of the doctrines of Jesus compared with those of others". He was elected a Fellow of the American Academy of Arts and Sciences in 1807. Van der Kemp and Mappa wrote in an early stage (1807?) about digging a canal between Albany and Buffalo, the work on the Erie Canal started in 1817. The state received 100,000 acres (400 km2) from Holland Land Company.
The governor of the state of New York, Dewitt Clinton, named him the most scholarly man in the U.S.A. In 1820 he received an honorary degree from Harvard University, and was elected a member of the American Antiquarian Society in 1822. Van der Kemp started to translate the 17th century records in English, but his translation in the New York State Library went up in smoke in 1911. Van der Kemp began to write his autobiography, but died of cholera on September 7, 1829 at the age 77 and is buried in the Olden Barneveldt cemetery Trenton, N.Y. Harvard University in Cambridge, Massachusetts received his library.