Gardner's basket with 2:53 remaining gave American a 44-42 lead, but Mount St. Mary's Khalid Nwandu answered with a 3-pointer for a one-point lead with 2:23 left. Neither team would score again until Gardner's heroics.
Mount St. Mary's Junior Robinson missed the front end of a 1-and-1 with 15 seconds to go.
Gardner was 8 of 13 from the field and finished with 17 points for American (7-4). Jesse Reed added 10 points.
Andrew Smeathers and Gregory Graves had eight points apiece for Mount St. Mary's (3-6). Taylor Danaher grabbed 12 rebounds, five on the offensive end, as the Mountaineers held a 35-27 advantage on the glass. Mount St. Mary's only made 17 of their 50 field goals (34 percent).