He had married Abigail Morgan Woodnutt in 1874 and they had three children, Thomas W., Clement W., and Mrs. Forest Willard.

Miller was elected Governor of Delaware in 1912 by defeating Thomas M. Monaghan, the Democratic Party candidate.

1912 was a Democratic sweep, except for Miller's narrow victory, perhaps attributed to the fact that Monaghan, his opponent, was a Roman Catholic.

During his term ferry service was initiated between New Castle, Delaware, and Penns Grove, New Jersey, and the Lewes and Rehoboth Canal was built.

The major event, however, was the outbreak of World War I in Europe and the effect in Delaware of the rapid expansion of demand for gunpowder.