For nearly four hundred years after its foundation, the the Shire had been part of the lands of Arthedain, and under the rule of that land's King. Ultimately, Arthedain fell to the forces of Angmar, and its last King, Arvedui, was lost, leaving the Shire-hobbits without a ruler. They remedied this by choosing a new leader from among themselves, Bucca of the Marish, who was given the title thain, a word simply meaning 'chief'.
Bucca and his descendants, a family known as the Oldbucks, served as Shire-thain for twelve generations. The twelfth Thain, one Gorhendad Oldbuck, left the Shire and crossed the River Brandywine to found Buckland. With his departure, the Thainship passed to a new line, the Tooks, and specifically to Isumbras Took I.
From Isumbras I, the line of the Took Shire-thains continued unbroken to the time of the War of the Ring and beyond. The last recorded Thain was Faramir Took I, the thirty-fourth to hold the title of Shire-thain. The fact that both he and his father Peregrin Took are recorded as the first of that name shows that there must have been more Thains after this time, and the tradition clearly lasted well into the Fourth Age.