Developing B cells can be positively or negatively selected by self-antigens, but the mechanisms that determine these outcomes are incompletely understood. Here, we show that a B cell intrinsic switch between positive and negative selection during ontogeny is determined by a change from Lin28b to let-7 gene expression. Ectopic expression of a Lin28b transgene in murine B cells restored the positive selection of autoreactive B-1 B cells by self-antigen in adult bone marrow. Analysis of antigen-specific immature B cells in early and late ontogeny identified Lin28b-dependent genes associated with B-1 B cell development, including Arid3a and Bhleh41, and Lin28b-independent effects are associated with the presence or absence of self-antigen. These findings identify cell intrinsic and extrinsic determinants of B cell fate during ontogeny and reconcile lineage and selection theories of B cell development. They explain how changes in the balance of positive and negative selection may be able to adapt to meet the immunological needs of an individual during its lifetime.