Henry Flagg French was born on August 14, 1813, in Chester, New Hampshire.
He was an American lawyer, judge, postmaster, agriculturalist, inventor, and writer. He first studied law at the law office of his father the Honorable Daniel French (1769–1840), who was attorney general of New Hampshire and a judge on the state’s Supreme Court and then attended Harvard Law School. In 1834, he was admitted to the bar and he practiced law in Chester for five years, until his father's death in 1840. In 1839, he succeeded his father as postmaster in Chester. He was a county solicitor from 1838 to 1848 and a bank commissioner from 1848 to 1852. From 1855 to 1859 he was the justice of the court of common appeals and district attorney for Suffolk County from 1862 to 1865. He was president of the Rockingham Agricultural Society (1852-1859) and the Massachusetts Agricultural College (1865-1866). In 1852, he received an honorary M. A. degree from Dartmouth College. In 1859, he opened a law office in Boston and in 1862, he was appointed assistant district attorney, a position he held until 1865. In 1876, he was appointed by President Grant as 2nd assistant secretary of the United States Treasury, where he served until 1885. French was a very prolific writer and wrote hundreds of articles for several agricultural journals. He also wrote a book titled "Farm Drainage" which described a particular type of drain called a "French Drain".
In 1838, he married Anne Richardson (1811–1856) and in 1859, he remarried Pamela Mellen Prentiss (1821–1895). He died on November 29, 1885, in Concord, Massachusetts.