Identifying factors that affect the self-assembly of the amyloid-β peptide (Aβ) is of utmost importance in the quest to understand the molecular mechanisms causing Alzheimer's disease (AD). Ca2+ has previously been shown to accelerate both Aβ fibril nucleation and maturation, and a dysregulated Ca2+ homeostasis frequently correlates with development of AD. The mechanisms regarding Ca2+ binding as well as its effect on fibril kinetics are not fully understood. Using a polymerization assay we show that Ca2+ in a dynamic and reversible manner enhances both the elongation rate and fibrillar stability, where specifically the "dock and lock" phase mechanism is enhanced. Through NMR analysis we found that Ca2+ affects the fibrillar architecture. In addition, and unexpectedly, we found that Ca2+ does not bind the free Aβ monomer. This implies that Ca2+ binding requires an architecture adopted by assembled peptides, and consequently is mediated through intermolecular interactions between adjacent peptides. This gives a mechanistic explanation to the enhancing effect on fibril maturation and indicates structural similarities between prefibrillar structures and mature amyloid. Taken together we expose how Ca2+ levels affect the delicate equilibrium between the monomeric and assembled Aβ and how fluctuations in vivo may contribute to development and progression of the disease.