Vaccine adjuvants play important roles in shaping the humoral response to immunization. Here, we analyzed mechanisms of action of a clinically relevant combination adjuvant strategy, where phosphoserine (pSer)-tagged immunogens bound to aluminum hydroxide (alum) adjuvant, promoting prolonged antigen release to draining lymph nodes, are combined with a saponin nanoparticle adjuvant termed SMNP, which alters lymph flow and antigen entry into lymph nodes. When used with a stabilized HIV envelope trimer antigen in mice, this combined adjuvant approach promoted substantial enhancements in germinal center and antibody responses relative to either adjuvant alone. Using single-cell RNA and B cell receptor sequencing, we found that the alum-pSer/SMNP combination augmented the clonal expansion and diversity of the germinal center B cell repertoire, coincident with an increased proportion of S-phase germinal center B cells and expression of positive selection markers. Moreover, we found that the combination adjuvant approach, but not alum-pSer delivery or SMNP alone, promoted accumulation of intact antigen on follicular dendritic cells, reflecting integrated effects of slow antigen delivery and altered lymph node uptake. Genetic ablation of expression by follicular dendritic cells eliminated antigen accumulation and hampered the antigen-specific germinal center response, supporting antigen delivery to these cells as a key mechanism of the improved response elicited by this combination adjuvant. These results demonstrate how adjuvants with complementary mechanisms of action affecting vaccine biodistribution and kinetics can enhance humoral immunity.