Mott was charged with second-degree murder and grand theft auto in November 2017. In exchange for his guilty plea, prosecutors have dropped the grand theft auto charge.
Prosecutors said Mott, who was 15 at the time, stabbed and shot his grandmother, Kristina French, and buried her in a shallow grave. He was arrested in Buffalo, New York, while attempting to cross in to Canada, authorities said.
Mott's father and girlfriend reportedly returned home from a vacation Nov. 22 and were not picked up at the airport by French as previously discussed. The couple took an Uber to the teen's father's Neptune Beach home and found it was ransacked and guns were taken from the gun safe, authorities said.