Born in Andover, Maine to Sylvanus and Mary (Merrill) Poor, he was the first of his family to attend college, graduating from Bowdoin in 1835. He joined his uncle’s law firm, being called to the bar in 1838. Later Henry and his brother John established a law practice in Bangor, Maine. By investing money in Maine’s growing timber industry, the Poor brothers made a fortune. John Poor became a minor railway magnate in association with the European and North American Railway, and was heavily involved in the building of the Maine rail network. In 1849, John purchased the American Railroad Journal, of which Henry became manager and editor. In 1860, Henry Poor published History of Railroads and Canals in the United States, an attempt to compile comprehensive information about the financial and operational state of U.S. railroad companies. He later established H.V. and H.W. Poor Co. with his son, Henry William, and published annual updated versions of his book. Standard & Poor’s traces its history back to this publication.