Did Farragut know where Mobile Bay's mines were?

Did Farragut know where the minefield was before ordering the fleet through it?

Yes. He knew the planned safe route and the minefield's position, then chose to cross after judging most of the older mines waterlogged or ineffective.

The original instructions kept the column east of the known minefield, so its position was not a surprise. After Tecumseh's loss and Brooklyn's halt, Farragut used visible conditions, reconnaissance, navigation experience, and ordnance knowledge to judge that most remaining mines had sunk or become unreliable. The decision still carried real danger because at least one mine had just worked. The answer reinforces that his advance was a conscious risk calculation rather than an accidental entry into unknown water.