Emmanuel Maguire Underdown was the eldest son of Emmanuel Underdown of Sidmouth in Devon. He married, first, Selina Pollard, and secondly, Marie Ehrenfest.
He was called to the bar on 6 June 1861. He became a barrister and Q. C. M. He was an expert on copyright and author of A Treatise on the Law of Art Copyright.
In 1885 he was living at 35 Onslow Gardens, and working at 3 Kings Bench Walk, Temple, London, EC.
According to his obituary, Underdown was a 'barrister, artist, musician and friend of Whistler'. He took the chair at the Criterion dinner in honour of Whistler on 1 May 1889. He was also the lawyer for the Society of Authors, of which Whistler's biographer, Joseph Pennell was a member.
Foster, Joseph, Men at the Bar: a biographical hand-list of the members of the various inns of court, including Her Majesty's judges, London, 1885; Obituary, The Lady, 23 April 1913; Who was who: a companion to Who's who, Vol. 1, 1897/1915, London, 1920-21.