Mild traumatic brain injury (mTBI) accounts for over 1 million emergency visits each year. Most of them stay in the emergency department for a few hours and are discharged home without a specific follow up plan due to their negative clinical structural imaging. Advanced magnetic resonance imaging (MRI), particularly functional MRI (fMRI), has been reported as being sensitive to functional disturbances after brain injury. In this study, a cohort of 12 mTBI patients were prospectively recruited from the emergency department of our local Level-1 trauma center for an advanced MRI scan at the acute stage. Sixteen age- and gender-matched controls were also recruited for comparison. Both group-based and individual-based independent component analysis of resting state fMRI (rsfMRI) demonstrated reduced functional connectivity in both posterior cingulate cortex (PCC) and precuneus regions in comparison with controls, which is part of default mode network (DMN). Further seed-based analysis confirmed reduced functional connectivity in these two regions and also demonstrated increased connectivity between these regions and other regions of the brain in mTBI. Seed based analysis using the thalamus, hippocampus and amygdala regions further demonstrated increased functional connectivity between these regions and other regions of the brain, particularly in the frontal lobe, in mTBI. Our data demonstrates alterations of multiple brain networks at the resting state, particularly increased functional connectivity in frontal lobe, in response to brain concussion at the acute stage. Resting state functional connectivity of the DMN could serve as a potential biomarker for improved detection of mTBI in the acute setting.