The son of Thomas Truebody Thomason, a British cleric in Bengal from 1808, and his first wife Elizabeth Fawcett, he was born on 3 May 1804 in Little Shelford.

He was educated in England from 1814, at Aspenden Hall School, Hertfordshire, where he knew Thomas Babington Macaulay, living with his paternal grandmother Mrs Dornford, and Charles Simeon.

[2] In 1818 Thomason became a pupil in 1819 at Stanstead Park, near Racton in Sussex, of George Hodson, who was tutoring Albert Way, son of Lewis Way in what became a small class of six boys that included Samuel Wilberforce.

He held numerous positions there, including magistrate-collector, settlement officer in Azamgarh (1832–37), and foreign secretary to the government of India (1842–43).

By 1853 he had also established a system of 897 locally supported elementary schools in centrally located villages that provided a vernacular education for children throughout the region.