William Hardell was a Mayor of London and a Magna Carta surety. He was the only commoner whose signature appears on the Magna Carta.

He was appointed Sheriff of the City of London in 1207 and elected Mayor of London (a century later known as Lord Mayor of London) in 1215.

After the sealing of the Magna Carta by King John in 1215, he was appointed to be one of the Enforcers, sometimes called Sureties, of the Magna Carta. The list of enforcers does not appear on Magna Carta itself and the first surviving list of these was by Matthew Paris the chronicler of St Albans.

References: Oxford Dictionary of National Biography via Wikipedia

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Excerpt from An Historical Essay on the Magna Charta of King John, by Richard Thomson, London, 1829, p. 293 (available on Google Books):

Of WILLIAM DE HARDELL, the Lord Mayor of London, the information now extant is neither so particular nor yet so authentic, as that which has been brought forward concerning the other Barons; and it has even been conjectured that the name has been corrupted from that of Mauritius de Londonio, or Maurice de London. The Armorial Ensigns, which are commonly assigned to William de Hardell, are Vert, a Fesse flory and counterflory, Or; but Richard, or Ralph Hardell, a Draper, who was Mayor for several successive years in the time of Henry III., is stated to have borne Or, a Bend between two Cottises, Sable: but armorial bearings for a Civil Officer of so early a period, should be received with considerable caution.

There do not appear to be any particulars extant, from which may be compiled an authentic account of William de Hardell, but he probably was one of the same family as that William, who was Clerk of the Wardrobe in 1245-46, the 30th of Henry III.; Wide Madox's History of the Exchequer, Chap. xxii., pages 621, Note a, 622, Note k, Chap. xxiii. page 674, Note w, 690, Note d. The same authority also mentions Ralph Hardell, the Lord Mayor of London, who was summoned to the King's council at Merton, concerning a disputed Taillage of the City: vide Chap. xvii. page 491; see also Chap. xxii. page 606, Chap. xxiii. page 632. A roll of the 55th of Henry III., 1270-71, likewise quoted by the same authority, Chap. xvii, page 516, Note b, mentions one Juliana Hardel; and in 1246-47, the 31st of Henry III., John Hardel was elected on the King's writ to be Keeper of the Die of the Mint of London. Wide Chap. xxii, page 604, Note f. An eminent family also of this name held several estates in Essex, as the Inquisitiones Post Mortem, for the 13th of Edward I.—1284-85–No. 28, record the possessions of a Laurencius Hardell; and some account of the family after this period, will be found in Morant's History of Essex, Vol. i. pages 276, 288,320.