I played a few multiplayer games with a friend this weekend against computer opponents and noticed that I didn't receive an achievement I know I should have earned. It was "Win a game as the Americans on any difficulty" and I was for sure playing as the Americans. The only reason I can think it might not have counted is that we were teamed up against the computers who were free-for-all.
We played two normal games before that, and I'm curious if those even counted towards my achievement progress. There is at least one multiplayer-specific achievement I earned then, so you would think everything else would count as well, right?
Best Answer
Playing with in-game modifications enabled will disable the ability to get achievements. I haven't found anything stating that simply playing multiplayer will prevent you from getting achievements but the achievements in Civilisation 5 are very specifically worded.
As you earned a multiplayer achievement during your play time I would go with you (obviously) having modifications off but the single-player achievements only taking effect in single player.
Update: found something, this is an archived post on the Steam forums complaining about lack of features in Civilisation 5 multiplayer.