"mentioned beforehand" is correct and is a common phrase used in written English.
It is usually used to refer to an idea, point, or fact that was brought up or mentioned earlier in a conversation or piece of writing. For example, "We discussed the possibility of implementing the new policy during the meeting, as mentioned beforehand by Sarah."
Klopp had mentioned beforehand that, while he respected Chelsea's array of world-class players, Liverpool had some of their own for their opponents to worry about.
The quantitative results showed that our approach outperformed the methods mentioned beforehand.
Some core institutional features of the complicated German system have to be mentioned beforehand.
However the main findings for the Piraeus' port, mentioned beforehand, are still valid.
As mentioned beforehand, we performed several test drives in order to capture data which is required to verify our approach towards trajectory estimation.
Barco Escape is already working with Jerry Bruckheimer to develop several projects, and he's also spoken of involvement with "two of the biggest action directors currently in Hollywood"—one possibly mentioned beforehand in this article.
We mention beforehand that some of the common notations will be used throughout this paper.
Newspapers barely mentioned it beforehand.
Here, she is presented as having an affair with Kennedy during his run for the White House and after, being involved with Sam Giancana, a Mafia boss, at the same time (not to mention, shortly beforehand, Frank Sinatra).
Finally, an oxy-regenerative flat-flame burner that transfers homogeneously radiant heat to the load, combining the two beforehand mentioned technologies that are used mainly for high capacity burners (higher than 0.5-1 MW).
The extended essay was introduced in 1978, but creativity, action, service (CAS), although mentioned in guides beforehand, was not specifically identified in the guide until 1989.
