Is there a tool to check citations over time across disciplines? I'd like to see how a given paper has impacted its own discipline and then spilled over to others. 

Let's take Nash's paper on Equilibrium points in n-person games as an example. This work has eventually yielded him a Nobel prize and has +6k citations on Google Scholar. 

I'm assuming it must have been first cited primarily by other mathematics papers; but over time, scholars from other disciplines would increasingly cite it.

Are you familiar with any tool/method that would provide this kind of information? It would be the most useful if it's possible to visualize the data.
You may try Microsoft Academic for that. They have an interesting semantic-web approach to finding answers to your queries and they also aggregate a lot of context information. This includes citations that can be filtered in various ways.

Have a look at this search regarding your questions's example: https://academic.microsoft.com/paper/2067050450/citedby/search?q=Equilibrium%20points%20in%20n-person%20games&amp;qe=RId%3D2067050450&amp;f=&amp;eyl=Y%3C%3D1956&amp;orderBy=0 (you were right: initially, most citations were from mathematics).