What program(s) did Riot use to make the environment in league?

Well, you can dig a little bit into the Riot Polycount contest that is already over, they are fan made assests, but the pipeline should be mostly the same, aside from the whole testing with Riots own client for tests. Software doesn't matter really, but it's mostly Zbrush/Mudbox, photoshop (or any 3d paint software like well, Zbrush, Substance Painter,Mudbox, Mari etc.), and any 3d package (like Maya) that you like.

http://www.polycount.com/forum/showthread.php?t=142258 http://www.polycount.com/forum/showthread.php?t=141494

And this should be in the Art forum.