Ben Garner had only been at the helm of the League One side for six months.
They have taken only two points from their last five League One matches to slip to 17th.
The owner of the Addicks said the search for a successor was underway.
"In his time at the club, the club have achieved some notable results, including big wins against some of the better teams in the league," Sandgaard said.
"Unfortunately, we haven't been able to sustain a consistency of results and our league position and recent form have left us in a situation where we had to act to give ourselves the best chance to get the most from this season as well as building a positive and winning environment for future years
The Addicks were booed off by their fans after Friday's home loss to Cheltenham.
After the game the former Bristol Rovers boss told the South London Press that sacking managers would not solve the club's "cultural" problems.
If you want, you can change the manager every few months. It is more than that. The issue is a cultural one. There is a mentality issue. It's not easy to change but that's what we need to do.
I would like to do that. I'm happy to do that. If I get the chance to do that, I will be successful.
Despite guiding the Addicks to a Carabao Cup fourth-round tie against the Seagulls, he only won three of his last 16 league games. In the month of October, they all came in a row.
In the second round of the FA Cup, the Addicks will play League Two Stockport.
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