What's the ground like? If that's sodden now, probably best to leave it and deal with it in the spring, when the ground is more firm.
Is it fenced? Grazing with cattle is always good for getting on top of longer grass, provided the ground can take their feet.
If you mow it, can you get it baled and wrapped, and use or sell it? Otherwise would suggest you not mow it but, if you do anything now, top it with a flail topper, which leaves a mulch that will put nutrients back into the ground. Mowing and leaving cut long grass to rot will give you more problems than leaving it as is.