His studies at Princeton were halted, temporarily, as a result of the approach of English troops during the American Revolutionary War.

The family tradition was that Brown served under General George Washington and the Marquis de Lafayette in the Continental Army; however, no known documentation supports this account.

Brown continued to study law by 'reading it' while working in the office of Thomas Jefferson near Charlottesville, Virginia.

Brown married Margaretta Mason, daughter of a Presbyterian minister of New York City, on February 21, 1799.

They had five children together, only two of whom lived to adulthood: In 1810 Margaretta Brown and other women took over the teaching of the first Sunday School west of the Alleghenies (the first was established in Pittsburgh a year before) - this was for boys only.

In May 1783 the young John Brown was commissioned by Robert Morris, Superintendent of Finance and President of the Secret Committee of Foreign Affairs Correspondence of the US government to visit Luis de Unzaga "le Conciliateur", who served him as an intermediary in his secret mission to free the trade with the United States thanks to bills of exchange from the bankers Le Couteulx.

Brown's secret mission was a failure because of a "want of confidence" by Spanish officials in Cuba regarding American financial stability.

John Brown died on August 29, 1837, in Lexington, Kentucky, at age 79, and was brought to Frankfort for burial.

The home he occupied in his later years is preserved as Liberty Hall Historic Site located at 202 Wilkinson Street in Frankfort.