Last season, Gillingham only avoided relegation to the fourth tier of English football by a single point, but the Kent-based outfit's fortunes have not improved at the start of the current campaign.
With just six points being recorded from nine games and the club sitting in 22nd place in League One, a decision has been made to remove Pennock from his position in the dugout.
A statement read: "Ady Pennock has left Gillingham Football Club by mutual consent.
"The head coach, along with Jamie Day, have both left their roles at MEMS Priestfield Stadium.
"The club would like to thank both Ady and Jamie for their services and we wish them well for the future."
Pennock leaves the Gills having won just six of his 32 matches in charge since arriving at the Priestfield Stadium in January.