He represented Glengarry as a Liberal-Conservative member of the House of Commons of Canada from 1879 to 1882.
He served as president of the Montreal Board of Trade and vice-president of the Merchants' Bank of Canada.
With his brother Hugh, he established a firm involved in shipping and grain processing.
Five years after his death in 1898, his widow Charlotte Adelaide (Mair) McLennan established a church and parish hall in his memory.
Located on part of McLennan's former Ridgewood estate, St. John the Evangelist Anglican Church remains open to this day, surrounded by the Glengarry Campground of the St. Lawrence Parks Commission in South Glengarry, Ontario.