Where it dose have some cleaning abilities...depending on how dirty your leather is I would clean it first with something else....diluted Woolite or APC with water. Then yes you can use NL on leather...not so much as a conditioner but has a protectant...I use it all the time on mine in between leather conditioning
If it's a late model vehicle, the chances are that it has a coated seat and then the answer would be yes If it is an older model or a model with an uncoated seat where the leather should be "fed" with an oil or lanolin then a product like Leather Stuff would be better suited and a cleaner like APC would be needed before hand.