The win gave Greeneville (4-5) the sweep and pushed the K-Mets’ losing streak to four.
In the final inning, Mathis led off with a walk, advanced to second base on a sacrifice bunt by Jack Mayfield and scored on an error on a throw and missed catch between K-Mets second baseman Jeff McNeil and catcher Adrian Abreu.
Kingsport (2-7) scored its lone run in the fifth off an Amed Rosario home run to center field.
Greeneville answered in the bottom of the frame when Wallace Gonzalez hit a solo shot of his own.
Mathis, Mayfield, Ariel Ovando and Gonzalez had two hits apiece for the Astros.
Kingsport — which struck out 25 times — got a team-best 3-for-6 effort from Rosario. Victor Cruzado added two hits and walked twice.
The Astros used six pitchers with Christian Garcia (1-0) picking up the win. The K-Mets’ Luis Rengel (0-1) worked 3 1-3 innings and was stuck with the loss after giving up the unearned run in the 15th.
Kingsport returns home Saturday for a three-game set against the Appalachian League-leading Elizabethton Twins. First pitch is set for 6 p.m. at Hunter Wright Stadium.