There are 11 US Post Offices in Grant County serving a total of 47,637 residents equating to 4,331 residents per post office. It's estimated that approximately 76,034 packages are handled across the 11 county post offices annually. Grant County is a county located in the U.S. state of Indiana. At the time of the 2010 census, the population was 70,061. The county seat is Marion. Important paleontological discoveries, dating from the Pliocene epoch, have been made at the Pipe Creek Sinkhole in Grant County.