Practical advances in Ni-catalyzed Suzuki-Miyaura cross-coupling (SMC) have been limited by a lack of mechanistic understanding of phosphine ligand effects. While bisphosphines are commonly used in these methodologies, we have observed instances where monophosphines can provide comparable or higher levels of reactivity. Seeking to understand the role of ligation state in catalysis, we performed a head-to-head comparison study of Ni SMCs catalyzed by mono and bisphosphine precatalysts using six distinct substrate pairings. Significant variation in optimal precatalyst was observed, with the monophosphine precatalyst tending to outperform the bisphosphines with electronically deactivated and sterically hindered substrates. Mechanistic experiments revealed a role for monoligated (P1Ni) species in accelerating the fundamental organometallic steps of the catalytic cycle, while highlighting the need for bisligated (P2Ni) species to avoid off-cycle reactivity and catalyst poisoning by heterocyclic motifs. These findings provide guidelines for ligand selection against challenging substrates and future ligand design tailored to the mechanistic demands of Ni-catalyzed SMCs.