Also in 1849 Mackenzie became secretary of Charles Cochrane's National Philanthropic Association. The National Philanthropic Association commissioned Mackenzie to write "A Plea for the Very Poor" (1850), on the plight of the 'very poor' in the aftermath of the ‘hungry forties’ in London, but also including reports of the Highland Potato Famine and of an Gorta Mór, the contemporaneous Great Famine in Ireland.