William Reilly was born in Scarva, County Down, and was educated at Woolwich. He entered the Royal Artillery and served in the Crimean War. In 1868 he was promoted to the rank of colonel and was made Companion of the Bath. He saw action in the Franco-Prussian War and was taken prisoner by the Prussians, later serving in the Zulu War. In 1885 he became major general. He died at sea.