Sullivan has just completed a loan spell at AFC Wimbledon and Mambo was most recently on loan with Shrewsbury.
Addicks manager Chris Powell said: "Those lads will be looking to play elsewhere. We are trying to be fair to them.
"John has been a terrific character, trainer and goalkeeper for us, but he needs to play regularly.
"He has just had a good taste of that and hopefully, whether it is at Wimbledon or elsewhere, he needs to get going.
"It is the same with Yado. He has had a taste of League Two and League One this year.
"The reports were good but I said to him once you get that week in, week out feeling of preparing and playing he would find it hard to play here."
- Sports & Recreation
- AFC Wimbledon