Provided are mixed carrier, multivalent pneumococcal conjugate compositions comprising 21 different pneumococcal capsular polysaccharide-protein conjugates, wherein each of the conjugates includes a capsular polysaccharide from a different serotype of Streptococcus pneumoniae conjugated to either tetanus toxoid (TT) or CRM197, wherein the Streptococcus pneumoniae serotypes are selected from 1, 3, 4, 5, 6A, 6B, 7F, 8, 9N, 9V, 10A, 11 A, 12F, 14, 15B, 18C, 19A, 19F, 22F, 23F, and 33F, where the capsular polysaccharides of two of serotypes 1, 3, and 5 and one or both of serotypes 15B and 22F are conjugated to TT and the remaining capsular polysaccharides are conjugated to CRM197. Also provided are methods of producing the mixed carrier, multivalent pneumococcal conjugate compositions and methods of using the same for prophylaxis against Streptococcus pneumoniae infection or disease in a subject.