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. For instance, in 1829 he was probably one of the instigators of the murder of Alexander Griboyedov by Persian mob in Teheran.
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.
This is the second edition of his report and the first that includes a map of “Sketch Acquisitions of Russia since the Accession of Peter 1st to the Throne.”