Emergency services were called shortly after 6am to reports of a concern for safety on the southbound carriageway between junctions 8 and 9.
Paramedics from South East Coast Ambulance Service attended but the woman was pronounced dead at the scene.
Her next of kin have been informed.
Surrey Police said it has referred itself to the Independent Office for Police Conduct (IOPC) because of the prior contact.
No arrests have been made and an investigation into the circumstances of her death is ongoing.
The motorway was closed between junctions 8 and 9 for several hours, causing major disruption.
The northbound side reopened shortly after 8am, while lanes 3 and 4 southbound reopened at 2pm.
Lanes 1 and 2 have now reopened.
Police thanked drivers for their patience during the disruption.
