Johnson is facing prison time after being found guilty of sexual activity with a child at Bradford Crown Court on Wednesday.
The 28-year-old was only sacked by Sunderland earlier this month after pleading guilty to one count of sexual activity with a child and one count of grooming, 10 months after he was charged.
"It's been very difficult to judge what is actually going on because you want to know all the facts and you want to keep your cool until the judgement has been made," Allardyce said.
"Now that the judgement has been made, we all feel extremely let down by what has happened and by what Adam has done. I certainly feel a lot of sympathy for the victim and the family.
"Hopefully that has given them some peace and they can get on with their lives. The decision on his punishment will be given by the judge, and he has to accept that and we all have to move on."