Lewis Wilkieson Johnstone (10 April 1862 – 9 March 1936) was a Conservative member of the House of Commons of Canada.
He was born in Sydney, Nova Scotia and became a physician and surgeon.
He was also a descendant of David Mathews, the Loyalist Mayor of New York City under the British during the American Revolution.
Johnstone attended King's College at Windsor, Nova Scotia, then in 1886 graduated in medicine from Bellevue Hospital Medical College.
Johnstone was defeated in the 1935 election by Daniel Alexander Cameron of the Liberal party.