Gram-negative bacteria use the conserved chaperone/usher pathway for assembly and secretion of a superfamily of virulence-associated surface structures. P and type 1 pili (fimbriae) are prototypical organelles assembled by this pathway in uropathogenic E. coli. These hair-like surface fibers mediate initial binding to and subsequent invasion of the urinary tract. Protein secretion and pilus biogenesis across the bacterial outer membrane are poorly understood. We captured a pilus assembly intermediate corresponding to the type 1 pilus tip fiber during secretion through the outer membrane FimD usher. 3D reconstruction of cryo-EM images revealed that the translocating usher is a dimer, with a single pilus tip fiber emerging from one of the two available usher pores. The location of the chaperone-subunit complex at the periplasmic face of the assembly intermediate likely explains the requirement of a twin pore for asymmetrical subunit translocation and pilus assembly.