The Marksmen - currently bottom of the National League North - announced the news following a 2-1 win over fellow strugglers Warrington Town.
The win leaves them only a point from safety in a tight race for survival that sees the bottom six sides separated by just a single point.
Former Manchester City and Swindon midfielder Horlock had been at the club almost four years and famously led them to the Southern League Premier Central title last season.
A statement on the club's website reads: "The club regret to announce the immediate resignation of manager Kevin Horlock, following this afternoon's 2-1 win over Warrington Town.
"Kev guided the club to four straight Suffolk Premier Cup triumphs, a record run to the FA Trophy Quarter-Finals as well as making the first round proper of the Emirates FA Cup and most notably last season's Southern League Central Premier Division triumph.
"All at the club would like to place on record it's sincere thanks to Kev for all his done for the club.
"Further announcements regarding the manager's position will be communicated in due course."