Halliday was the eldest son of John Halliday of Yard House, Taunton and his wife Mary Welman, daughter of Isaac Welman of Poundisford Park, Somerset.
He founded the banking firm of Halliday and Co. in Lombard Street in about 1770, and went into partnership with Sir John Duntze, Bt in 1776.
[1] Halliday contested Taunton on the interest of the Market House Society in the 1774 general election but was defeated.
Halliday did not stand at the 1784 general election because he hoped to be given a post as commissioner of taxes.
To this end he gave up his partnership in the banking house to remove any objection to the appointment, and worked hard to get an Administration supporter returned at Taunton.