He was elected a Scholar of the college in 1938, and edited the student magazine T.C.D.
He is perhaps best known for his famous summation of the plot of Samuel Beckett's Waiting for Godot: "... has achieved a theoretical impossibility—a play in which nothing happens, that yet keeps audiences glued to their seats.
Despite what may sound like a somewhat disparaging criticism, Mercier was one of the foremost Beckett scholars of his day, and wrote extensively about Godot.
He also wrote a critically acclaimed study of Beckett's work as a whole, Beckett/Beckett.
His last marriage (1974–1989) was to the Irish novelist and children's writer Eilís Dillon, who edited his posthumous book, Modern Irish Literature: Sources and Founders (Oxford, 1994).