Catalytic cross-coupling by transition metals has revolutionized the formation of C-C bonds in organic synthesis. However, the challenge of forming multiple alkyl-alkyl bonds in crowded environments remains largely unresolved. Here, we report the regioselective functionalization of olefins with sp3-hybridized organohalides and organozinc reagents using a simple (terpyridine)iron catalyst. Aliphatic groups of various sizes are successfully installed on either olefinic carbon, furnishing a diverse array of products with congested cores featuring C- or heteroatom-substituted stereocenters. The method enables access to valuable but synthetically challenging C(sp3)-rich molecules, including alicyclic compounds bearing multiple contiguous stereocenters through annulation cascades. Mechanistic and theoretical studies suggest a stepwise iron-mediated radical carbometallation pathway followed by outer-sphere C-C bond formation, which potentially opens the door to a broader scope of transformations and new chemical space.