I‘ll sit longer

[Silverobama]

I was with Thomas and we had a talk. He was going to leave but I still want to sit in the venue (drinking coffee) for a while. I told him "I'll sit longer; you leave first". I don't think this is natural. How can I express the idea naturally?

 

[emsr2d2]

Honestly, the whole thing is very unnatural. Let's take it a sentence at a time:

I was with Thomas and we had a talk.

I suspect you mean something like "I met my friend Thomas for a chat the other day".

He was going to leave but I still want to sit in the venue (drinking coffee) for a while.

Your tenses don't agree, as marked above.
You were probably in a coffee shop, not an exhibition hall or similar, right? "Venue" isn't the appropriate word here.
Why is "drinking coffee" in brackets? If you want to include the information, then include it in the main body of the sentence. If it's irrelevant, leave it out completely.

I told him "I'll sit longer; you leave first".

This is really odd! I'd go with something like "I said to him "I'm going to stay a bit longer. If you want to go, that's fine. I'll see you soon"."