Don't mention it is a modest reply to being thanked. Not at all, don't mention it.
It denies that the favour was worth thanking for, which is the polite English move. It sits with not at all and no trouble, and is a shade more formal than no problem.
1phrasedon't mention itA modest reply to being thanked.
“She thanked him twice and he said don't mention it.”
don't mention it is a phrase.
Practised in Small Talk & Everyday Phrases