Include "against" or leave out?

Is it correct to use against with mitigate as in this generic example?

Since this risk type is rapidly evolving, our department is working to clarify the definition of (product) to mitigate against this exposure in the future.

The original did not include "against" but a reviewer (not a communications person) suggested the insertion.

The use of against here doesn't add much to the meaning of the sentence.

The usage, as per Google NGrams, seems to agree.

I'd leave it out.

"Mitigate against" is generally not accepted as proper usage. You should just use "mitigate".

mitigate against Usage Problem

http://www.thefreedictionary.com/mitigate3

Incorrect: Dramatic action is required to mitigate against the devastating consequences of climate change.

Correct: Dramatic action is required to mitigate the devastating consequences of climate change.

http://aimpublishing.com/aimpub/word-usage/mitigate-mitigate-against

As John Lawler notes in a comment above, the impetus to add against after mitigate comes from confusion of mitigate for militate. The meanings of the two words aren't especially similar. Here is the entry for mitigate in Merriam-Webster's Eleventh Collegiate Dictionary (2003):

mitigate vt (15c) 1 : to cause to become less harsh or hostile : MOLLIFY {aggressiveness may be mitigated or ... channeled —Ashley Montagu} 2 a : to make less severe or painful : ALLEVIATE b : EXTENUATE

And here is the corresponding entry for militate:

militate vi (ca. 1600) : to have weight or effect {his boyish appearance militated against his getting an early promotion}

As you can see, the militate example is followed by against. In contrast, the mitigate example (for definition 1 of that term) doesn't follow mitigate with against, and in fact the example would sound quite bad with that addition. Moreover, none of the three single-word synonyms given for mitigate (namely, mollify, alleviate, and extenuate) is ever paired with against either.

Notwithstanding all of these points, the Eleventh Collegiate offers the following rather tendentious usage note regarding "mitigate against":

usage Mitigate is sometimes used as an intransitive (followed by against) where militate might be expected. Even though Faulkner used it {some intangible and invisible social force that mitigates against him —William Faulkner} and one critic thinks it should be called an American idiom, it is usu. considered a mistake.

In my opinion, the reason it is usu. considered a mistake is because it usu. is a mistake—including in the instance penned by the Olympian William Faulkner.

In the OP's example—

Since this risk type is rapidly evolving, our department is working to clarify the definition of (product) to mitigate [against] this exposure in the future.

—the sense of mitigate is probably the Eleventh Collegiate's definition 2(a) ("ALLEVIATE"). I see no reason whatsoever to add against after mitigate; nor can I find any rational basis for replacing mitigate with militate against. I recommend discarding the "American idiom" that your reviewer introduced and sticking with mitigate in its simple, unadorned, and correct form.