Son of a London merchant, he was born at Peckham. He was educated at St Paul's School and at Heidelberg University, which was famous as a school of law. Called to the bar at the Middle Temple in 1848, he joined the home circuit. At this time he also wrote for the press, and supplied special reports for the Morning Chronicle. Though not eloquent in speech, he was clear, accurate and painstaking, and soon advanced in his profession, passing many more brilliant competitors. He appeared for the claimant in the Shrewsbury peerage case in 1858, when Henry Chetwynd-Talbot, 3rd Earl Talbot was declared to be entitled to the earldom of Shrewsbury as the descendant of John Talbot, 2nd Earl of Shrewsbury; was principal agent for the United Kingdom on the mixed British and American commission for the settlement of outstanding claims, 1853–1855; and assisted in the prosecution of the Fenian prisoners at Manchester. In 1868, Hannen was appointed a judge of the Court of Queen's Bench. In many cases he took a strong position of his own, notably in that of Farrar v. Close, which materially affected the legal status of trade unions and was regarded by unionists as a severe blow to their interests. Hannen became judge of the Court of Probate and Court for Divorce and Matrimonial Causes in 1872. By virtue of being the Probate Judge, he became President of the Probate, Divorce and Admiralty Division by operation of the Judicature Acts when three courts became part of the new High Court of Justice. Here he showed himself a worthy successor to Cresswell Cresswell and Lord Penzance. Many important causes came before him, but he will chiefly be remembered for the manner in which he presided over the Parnell special commission. His influence pervaded the whole proceedings, and it is understood that he personally penned a large part of the voluminous report. Hannen's last public service was in connection with the Bering Sea inquiry at Paris, when he acted as one of the British arbitrators. In January 1891 he was appointed a Lord of Appeal in Ordinary with the dignity of a life peerage as Baron Hannen, of Burdock in the County of Sussex, but in that capacity he had few opportunities for displaying his powers, and he retired at the close of the session of 1893.
