I'm a Monorail Cast Member, here are the hours of operation:
Resort Monorail: 7am until 2 hours after the last park closes. This includes EMH (Extra Magic Hours). So let's say Hollywood Studios is open until 1am one day, the resort monorail will operate until about 2:45/3:00am.
Express Monorail: 8:30am until 1 hour after MK closes. This typically does not include EMH hours, because since EMH is for resort guests only, guests can take the resort monorail for Poly, Grand Flo, and Contemp, or busses in front of MK for all other resorts.
Epcot Monorail: 7:30am - 1.5 hours after MK closes.
Guests who drive over, or get a bus from a non Disney resort, who want to get to the Magic Kingdom before the express monorail opens, you can take the Resort Monorail. MK is the 3rd stop from the TTC (Transportation and Ticket Center).