Objective: Boxers receive numerous concussions and sub-concussive blows throughout their careers, which may have long-term consequences on cognition. Studying concussion-related changes in two prominent neural networks, the default mode network (DMN) and central executive network (CEN), may provide insight into the effects of this type of trauma. We hypothesized that functional connectivity (FC) among nodes of these networks would decline following a concussion, and that the degree of decline would correlate with cognitive outcomes. Participants and Methods: Ten professional boxers were scanned prior to a fight and 7 days after a boxing-related concussion. Resting state fMRI and DTI were obtained using a 3T Philips MR system. Neuropsychological data was collected using ImPACT on the days of the scans. DMN, CEN, and whole-brain (WB) FC were calculated using a seed-based approach to measure the degree of BOLD synchrony between each network node and the respective seeded regions. Results: No significant changes in the overall FC of the DMN, CEN, or WB were noted post-concussion, though WB showed a trending decline. Significant declines were noted between the seeded regions and individual nodes of the CEN and DMN on the left side of the brain. At baseline, both verbal memory and cognitive efficiency correlated positively with FC in select nodes within the DMN. Decrease in DMN FC was associated with a slowing of reaction time between baseline and post-concussion. Boxers with more professional experience had greater declines in FC among select nodes of the DMN. Boxers who received more head hits in the fight in which they were concussed had a greater decline in FC between nodes of the CEN. Conclusions: Mild boxing-related concussions cause decreases in the FC of neural networks, and are associated with declines in neurocognitive ability.