He was born in Perth, Scotland. His mother died shortly after his birth and his father was by turns a naval officer and a foot soldier. He was educated at the Caledonian Asylum, London, and at Brussels, but spent much of his early life in France. Coming to London in 1834, he engaged in journalism, working for the Morning Chronicle from 1835–1844 and then became Editor of The Glasgow Argus. He moved to the Illustrated London News in 1848 becoming Editor in 1852.

He published Songs and Poems (1834), wrote a History of London, Extraordinary Popular Delusions and the Madness of Crowds (1841), and a romance, Longbeard. He is also remembered for his Dictionary of Lowland Scotch. During his lifetime, his fame chiefly rested upon his songs, some of which, including Cheer, Boys, Cheer, were in 1846 set to music by Henry Russell, and had an astonishing popularity.

Mackay first visited and published his observations about America as Life and Liberty in America: or Sketches of a Tour of the United States and Canada in 1857-58 (1859). He returned to act as Times correspondent during the American Civil War, and in that capacity discovered and disclosed the Fenian conspiracy. He had the degree of LL.D. from the University of Glasgow in 1846. He was a member of the Percy Society. He died in London.

His daughter became known as the novelist Marie Corelli.

This article incorporates public domain text from : Cousin, John William (1910). A Short Biographical Dictionary of English Literature. London, J. M. Dent & Sons; New York, E. P. Dutton.