However, the writer, actor and producer, who in the past 10 years has achieved worldwide success with his lunchtime theatre concept, A Play, A Pie and A Pint, in Oran Mor in Glasgow's west end, has revealed he is suffering from motor neurone disease.
The 64-year-old, who has enticed the likes of Robbie Coltrane, Elaine C Smith and Bill Paterson to star in plays in his Oran Mor basement theatre, was told about the "catastrophic diagnoses" by his consultant a year ago.
MacLennan, a hugely colourful figure and the son of two Glasgow doctors who ignored their advice not to pursue a career in theatre, chose not to go public about his illness until now.
The news will shock the theatre world's thousands of writers, directors and actors he has worked with over the years, and the many thousands of theatre-goers he has delighted.
Motor neurone disease is a degenerative disorder of the cells that control muscle movement. There is no cure, though treatment can relieve symptoms and slow the progression.
Yet MacLennan, who is married to actress and Balamory star Juliet Cadzow, reveals in today's Herald that he will not lie down to the illness. He said he was determined to carry on working.
He said: "When you get what my neurologist called 'a catastrophic diagnoses' - he wasn't mincing his words - you've got to ask 'Will this define me?' At some point I guess it will, but for the moment, I'm going to carry on."
He added: "I think one of the things about getting this wretched illness is it focuses the mind on how lucky I've been with my work, my family and my friends."