Originally from Wales, Evans was in America and established as a surveyor and mapmaker in Philadelphia, Pennsylvania, from 1736; his first cartographical work is dated 1838. He is known as an associate of Benjamin Franklin and colonial governor Thomas Pownall.
In 1743, Evans accompanied the naturalist John Bartram on an expedition from Philadelphia, across the Appalachians, to Lake Ontario. The notes he made along the way, preserved for posterity in a copy made by Pownall, detail Evans’s musings on the nature of the geology and geography of the American landscape, pointing out the apparent historical influence of water on geomorphology. In this he was more than one hundred years ahead of his time.
However, Evans is now best remembered for his maps: A Map of Pennsylvania, New-Jersey, New-York, and the Three Delaware Counties (1749), and A General Map of the Middle British Colonies in America (1755). The latter was the most important that had yet been published in America, far more accurate and detailed than any previous general map of the area, and outstanding for its depiction of the trans-appalachian regions. Proof examples of the map were sent to, and used by, British general Edward Braddock, and were influential in his planning as he fought the French and Indian War.
To accompany his map, Evans published two controversial pamphlets: Geographical, Historical, Political, Philosophical and Mechanical essays (1755 and 1756). The critics, powers-that-be, and Evans each accused the other of treason, but it was Evans who was imprisoned. He was released on 8 June 1856, on a writ of habeas corpus because he was ill, but he died three days later in New York City.