In this paper we develop general formulas for the subdifferential of the pointwise supremum of convex functions, which cover and unify both the compact continuous and the non-compact non-continuous settings. From the noncontinuous to the continuous setting, we proceed by a compactification-based approach which leads us to problems having compact index sets and upper semicontinuously indexed mappings, giving rise to new characterizations of the subdifferential of the supremum by means of upper semicontinuous regularized functions and an enlarged compact index set. In the opposite sense, we rewrite the subdifferential of these new regularized functions by using the original data, also leading us to new results on the subdifferential of the supremum. We give two applications in the last section, the first one concerning the nonconvex Fenchel duality, and the second one establishing Fritz-John and KKT conditions in convex semi-infinite programming. Keywords Supremum of convex functions • subdifferentials • Stone-Čech compactification • convex semi-infinite programming • Fritz-John and KKT optimality conditions Mathematics Subject Classification (2010) 46N10 • 52A41 • 90C25