[2] From 1824 to 1835, he was attached to the East India Company's legation in Persia, at first in medical charge, and latterly as political assistant to the Minister, John Macdonald Kinneir, in which post he displayed great ability.
On 30 June 1835, he was appointed secretary of the special embassy sent to Tehran under Henry Ellis to congratulate Mohammad Shah Qajar on his accession to the Persian throne.
McNeill received permission to wear the Persian Order of the Lion and the Sun of the first class, and on his return home in the spring of 1836, he anonymously published a startling anti-Russian pamphlet, Progress and Present Position of Russia in the East.
In 1845, McNeill was appointed chairman of the Board of Supervision, entrusted with the working of the new Poor Law (Scotland) Act 1845, a post he occupied for twenty-three years.
It sharply criticised Lord Raglan's personal staff in the Crimea and Commissary-General Filder, and it led to many recriminations as officers sought to clear their names when the report was published in 1856.
Very unusually, the Commons, irritated by executive obfuscation, passed a resolution in 1857 calling for special honours and McNeill soon became a Privy Councillor and Tulloch was appointed a KCB.