I am curious, if I wanted to use a mosfet to control a different circuit that is using a different power source, I.E 6V going the Gate and 9V on the drain side for an N type, why do the grounds have to be common...

 

Because the gate control is referenced to the source which is one end of the controlled channel (the silicon between the source and drain).

Actually, the gate control is referenced to the substrate (the body of the chip) not the channel. But in most MOSFETs the substrate is connected to the source. This is usually shown on the symbol.

 

A more general answer is that any voltage, by definition, is the EMF between two points. There is no such thing as a single point voltage. Thus the voltage to drive the MOSFET gate must be referenced to something and that typically is ground or common. If the two grounds aren't common than the voltage on one ground isn't seen by the voltage on the other ground.

 

Is there an issue if I have both negative leads going into the same channel bank on a break board to ensure all components have the same common ground?