The Iron have lost all seven of their National League South fixtures so far and Borg becomes the first manager to leave his position this season.
Braintree are bottom of the table and he stood down following Saturday’s 4-0 defeat at Eastbourne Borough.
The Cressing Road board officially accepted his resignation on Sunday and now their search for a saviour has started.
The Essex club hope for a response on Tuesday night when they welcome Concord Rangers - but the club have not yet announced who will take the team for the important fixture.
Chairman Lee Harding says it’s difficult to say goodbye to a manager who has a long association with the club.
"I had high hopes for this season and will always be grateful for the way George stepped into the position after we have been let down last season,” he told the club’s website.
“George was the first to accept that it’s a results business, but the last six months have been so difficult for clubs to prepare with the delayed start to the season and then having to play behind closed doors making it difficult to attract players to the club.
“Despite the difficult position we find ourselves in, George will always be the manager that took the club to the Isthmian play-offs, Isthmian championship and National League South play-offs in three successive seasons.
“It is a legacy which we all still benefit from. I share the responsibility for the poor start to the season and I now look to re-start our campaign with a positive performance against Concord on Tuesday."