Mercenary is a perk available at the very beginning of the game for all heroes. You will start the game with 5000 bonus minerals, and 1 additional Talent points and increased nuke damage. Every major boss killed and Hive destroyed will give an additional 5000 minerals. Mercenary also permanently doubles your Hero's sight range, but it is unknown whether it doubles after all other sight increasing buffs, or the base vision only.
This perk is best paired up with aggressive nukes early game, and thus making it mandatory for one in the group to have it for faster pushing. Mid and late game nuking also works well as you'll have a lot of money banked up and will be able to clean big parts of bases fast.