Thomas Blore's background was in antiquarian draughtsmanship rather than architecture, in which he had no formal training. Charles Locke Eastlake, writing in 1872, believed that he had been apprenticed to an engraver, but other sources dispute this.
He illustrated his father's History of Rutland (1811), and over the next few years he made the drawings of York and Peterborough for John Britton's English Cathedrals, and drew architectural subjects for various county histories. In around 1822 Blore supplied the illustrations to T. F. Dibdin's Aedes Althorpianas. In 1823 he toured Northern England, making drawings for a work called the Monumental Remains of Noble and Eminent Persons. It was issued in parts with text by the Rev. Philip Bliss, and completed in 1826. Blore engraved many of the plates himself. In 1826, he was appointed surveyor to Westminster Abbey. In 1827 he was engaged to furnish plans for the chancel fittings of Peterborough. Shortly afterwards he was employed to restore Lambeth Palace, then in a state of near ruin. His work there included the construction of a fire-proof room for the preservation of manuscripts and archives.
Eastlake praised Blore's careful detail in his work at Westminster Abbey, adding "this was, in short, his great forte. He had studied and drawn detail so long and zealously that its design came quite naturally to him, and in this respect he was incomparably superior to his contemporaries.". Blore is most notable for his completion of John Nash's design of Buckingham Palace, following Nash's dismissal. He completed the palace in a style similar to, but plainer than that intended by Nash. In 1847, Blore returned to the palace and designed the great facade facing The Mall thus enclosing the central quadrangle. He also worked on St James' Palace in London, and a large number of other designs in both England and Scotland, including restoring Salisbury Tower at Windsor Castle.