Henry Miller Watts was born in Carlisle, Pennsylvania, the son of David Watts and the grandson of Revolutionary War generals on both sides of his family. He was educated in the best schools available at the time and entered the local Dickinson College with the class of 1824; his brother, Frederick Watts, had attended earlier, with the class of 1819. Following graduation, Henry studied law with Andrew Carothers, who also trained his brother and, in turn, had trained in the law office of the father of the two as a young man. Henry Watts passed the Cumberland County bar in 1827 and then, perhaps to escape the close professional family he had joined, traveled to Pittsburgh in the west of the state to set up his own practice.
After being in the area only a few months, Watts was appointed as a deputy Attorney General for Pennsylvania and served two terms before returning to build up his private practice. In 1835, he was elected as a state representative from Allegheny County and served three, one year terms. Exhibiting the kind of restlessness for pastures new that he would display his whole life, he left the area for Philadelphia and reopened his private practice there in 1838. In 1841, President Harrison named him as United States Attorney for eastern Pennsylvania and and served the full term. In 1857, his wanderlust struck again and he determined to take his wife and family to Paris for the purpose of enrolling his children in the schools of the city. He was back in Philadelphia when the Civil War broke out and in late 1862 he became one of the fifty founder-members of the Union League of Philadelphia — a patriotic club founded to support President Lincoln and still in existence today — on Broad Street. Despite this commitment to the union cause, he was off again in 1863, taking his eldest sons to study mining and engineering in Dresden. He made a second visit soon after, lasting eighteen months, touring other areas of Germany.
Back in Philadelphia, Watts was tapped in July 1868 as the United States ambassador to the Austrian Empire. This post had been left open since the recall of the previous envoy the summer before and in the interim the Senate had rejected seven of President Johnson's nominations and Horace Greely had turned it down. The Senate finally accepted Watts in August 1868 and he arrived in Vienna on September 25, 1868. He served less than a year, however, and was recalled in June 1869 when the new President Grant filled his place with John Jay. Returning to Pennsylvania, Watts now devoted himself to his practice and his business interests in iron and coal in places such as Lancaster County.
In 1838, Watts married Anna Maria Shoenberger of Pittsburgh and the couple had nine children. After a long and eventful life, Henry Miller Watts died on November 30, 1890 at the age of ninety-five.