"since I mentioned" is correct and usable in written English.
You can use it to introduce a fact that you previously spoke about. For example: "Since I mentioned it earlier, I'd like to reiterate that I am extremely passionate about conservation."
It's been some time since I mentioned it.
5.42pm GMT 42 min: Since I mentioned the possibility of the game becoming pretty bad, it's been very bad.
"I thought I might get a free lunch since I mentioned the restaurant in my book," McIntyre yuk-yuks to him.
3.06pm: Karlsson, who's been dropping faster than the value of my flat since I mentioned how well he was playing, holes for par on the eighth.
Why did you decide to speak out on Net neutrality? A. I have had an opinion on Net neutrality since I mentioned it in a book — effectively, but not by that name — a long time ago.
Ever since I mentioned this story to you, I have continued to ponder it, searching for more details, trying to come to a deeper understanding of my motives at the time.
I've also put the widget in this post for the startup PoliticalBase, since I mention them in the next paragraph.
And since, as I mentioned, there did, indeed, use to be a way that I would turn every single solitary sentence into a 'Dubya reference, could tie everything bad in the world to that guy..
I wrote about celery once before long ago; I don't know why, since as I mentioned in my piece Sunday in The Times Magazine, I had detested it.
But it has now emerged that the company, whose chair is an adviser to David Cameron, has been investigated nine times by the Department for Work and Pensions since 2005". I mentioned earlier that James Ball is working on a series of Reality check posts about the government's work experience programmes.
MR: Since as I mentioned earlier you are considered one of the great folk artists of the last twenty-five years, got any big plans for the next twenty-five years?
