The ale brand is owned by the UK brewer Greene King, which also owns Abbot Ale, IPA and Ruddles County.
Fallon, which won the account without a pitch, has been asked to broaden the brand's appeal to a wider target audience. The account was previously handled by WFCA.
Fallon has been briefed to develop a through-the- line campaign comprising outdoor, press, online and on/off trade.
Karina Wilsher, the Fallon managing director, said: "Old Speckled Hen is a truly unique brand that presents us with a real opportunity to shake up the category."