A mezzanine header connector includes a contact assembly having a pair of contact modules arranged back-to-back. The contact modules each have a dielectric holder holding a plurality of header contacts. The dielectric holder extends between a mating end and a mounting end and has an inner side and an outer side. The header contacts have mating segments exposed along the outer side at the mating end for termination to corresponding receptacle contacts of a mezzanine receptacle connector. The header contacts have terminating segments extending from the mounting end of the dielectric holder for termination to a circuit board. Each of the header contacts extend along a linear path between the corresponding mating segment and the corresponding terminating segment. The inner sides of the dielectric holders of the pair of contact modules abut against each other such that the header contacts face away from each other.