Don't mention it means you're welcome, it was nothing. You say it in reply to thanks to show that helping was easy: 'Thanks for carrying the box.' 'Don't mention it.' It sounds warm and a little more modest than you're welcome. People often add a reason: Don't mention it; I was going that way.
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Thanks for something. — Don't mention it.
- Thanks for carrying the box. — Don't mention it.
- I really appreciate the lift. — Don't mention it; I was going that way.
- Thank you for checking the address. — Don't mention it. I'm glad we caught the mistake.
What changed
First versionThanks for lending me your charger. — Don't mention about it.
RevisionThanks for lending me your charger. — Don't mention it.
We talk about something, so about slips in after mention: don't mention about it. But mention takes its object directly: mention it, mention the problem. Without about, the phrase is correct.
Don't mention it to Leo is a literal request: 'don't tell Leo about this'. As soon as there's a listener or a secret, the 'you're welcome' meaning disappears. My pleasure stresses that you enjoyed helping, and you're welcome is the neutral reply.
Your turn
Reply politely to thanks: Don't ___ it.
Show the answer
mention — Mention takes its object without a preposition: don't mention it. About is extra here, even though talk about makes it feel natural.
Your neighbour thanks you for watering their plants while they were away. Reply in one sentence with don't mention it.
Don't mention it — I was watering mine anyway.
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