Kinahan Cornwallis was born in London, United Kingdom of Great Britain and Northern Ireland, and attended school in Liverpool. He studied for the ministry and medicine before entering the British Colonial Civil Service. While in the civil service, Cornwallis spent two years in Melbourne, Australia. He also traveled over the years to the Philippines, Singapore, Ceylon, Egypt, Japan, Africa, South America, Canada, and the United States.

In 1860, Cornwallis joined the staff of the New York Herald. He worked for the Herald for about nine years. After leaving the Herald, Cornwallis became editor and owner of the Knickerbocker Magazine and the Albion. In 1886, he assumed control of the Wall Street Daily Investigator. During this time, Cornwallis was also engaged in the practice of law.

In 1858, Cornwallis wrote The New El Dorado (or British Columbia) with the intent of persuading the upper class to immigrate to British Columbia. In 1860, Cornwallis released Royalty in the New World (or The Prince of Wales in the New World. This work features full descriptions of the principal cities and towns, and chief natural features of British North America and the United States, together with accurate geographical and statistical information respecting the country generally, and a traveller's reflections upon the manners and customs of the people. On 16 November, the New York Herald included the following: "We are glad that Mr. Cornwallis, who accompanied the royal party throughout their tour, is about to supply a popular want, by the publication of a complete narrative of the royal progress. We feel assured that no one is more capable, both from literary talent and personal observation, of doing entire justice to the subject as a matter of history, and of producing a lively, agreeable, and entertaining book withal, than the gentleman to whom we refer. And there is no doubt the work will prove a pleasant souvenir of the New World travels of the eldest son of Queen Victoria, and have a large circulation, not only in this country, but on the other side of the Atlantic, where it is announced to appear simultaneously with the American edition."

Kinahan Cornwallis was father to Kinahan Cornwallis, the diplomat.