Note: avoid using subscript (2D) indexing on vectors. Prefer linear (1D) indexing. If C is a vector with 5 elements, C{5} will work whether or not it's a 5x1 or 5x1 cell array, whereas C{5,1} will error on a 1x5 cell array.
Note 2: "I have a 5-by-1 cell called C. I can extract the first matrix by C{1,1}{1,1};" The first element of C is C{1} (or C{1, 1} using subscript indexing). If you have to use C{1}{1} then the cells of your cell array are themselves cell arrays. That does not sound like it was intended from your description.
Anyway, If you want the max as a one liner:
M = max(reshape(C{1}{1}, 1, [])); %reshape the matrix into a vector before taking the max
However, I would recommend splitting the above in two for clarity
img = C{1}{1}; %get content of cell
M = max(img(:)); %get the max of all pixels
