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. In 1851, during the Highland Potato Famine – nearly as disastrous as the Great Famine of Ireland – he conducted a special inquiry into the condition of the western Scottish Highlands and Western Isles, during which he personally inspected twenty-seven of the most distressed parishes.