"I have mentioned" is correct and usable in written English. You can use it when you are referring to something that you already mentioned or brought up in conversation, or when you are citing a point or idea that you previously mentioned. Example sentence: I have mentioned that this project needs to be completed by this Friday, so please make sure it is finished on time.
(I have mentioned that on occasion).
I think I have mentioned Calvino.
As I have mentioned before, Duncan Smith has form.
Significantly, the two keepers I have mentioned are both foreign.
As I have mentioned before, these standards are primarily based on bird habitat quality.
Should I have mentioned the "work hard and follow your passion" mantra?
As I have mentioned before, we rely on Google AdWords as our primary source of customers.
Ludwig does not simply clarify my doubts with English writing, it enlightens my writing with new possibilities
Simone Ivan Conte
Software Engineer at Adobe, UK