Provided are a vehicle body front portion structure that enables an increase in vehicle body rigidity and strength while suppressing an increase in weight and a decrease in layout freedom, and a vehicle body assembly method. A vehicle body front portion structure (1) is provided with: a first weld portion (11) where a shield lower flange (4a) of a windshield lower (4) is abutted on a closed cross section of a front pillar (2), and where the shield lower flange (4a), an inner front flange (21a), and a rear end portion (5a) are spot-welded; and a second weld portion (12) where a general portion (5b) of a side member (5), a stiffener front flange (22a), and an outer front flange (3a) are spot-welded at a position forwardly of the first weld portion (11).