NOTE: In order to be ranked on the World Ranking leaderboard you must be playing on "Hard" difficulty.
In order to check the ranking on any mission you've played on, go to that specific mission. When the mission is opened up you may press "Y" to go to World Ranking (it'll give you rankings based on Global, Nearby, and Friends).
As soon as your rankings load up, back out to the globe screen and your achievement should pop immediately afterward.
Happy hunting!
EDIT: It seems that now that the game has a bigger playerbase, you must place at least in the 'Top 5000" or 'Top 10000' rankings (or that seems to be the assumption since Jimbo LaFleur received the achievement once being around Rank 4800 and also ranked around 7700.
Thank you to Jimbo LaFleur for providing this new information!