It happened when the woman, who has not been named, was enjoying a day out with her family at Bewl Water countryside park near Lamberhurst.
The woman, who is thought to be in her 40s and to live locally, is understood to have been taking shelter beneath a tree and was standing with her husband and daughter, who were both unhurt, when the lightning struck.
Staff at Bewl Water have been trained to respond to such incidents, and three of them assisted the woman at the scene and performed CPR for 30 minutes until emergency services arrived.
CPR [cardiopulmonary resuscitation] is a life-saving technique normally used when breathing or heartbeat has stopped.
Paramedics transferred the injured woman by ambulance to King’s College Hospital in London.
In a statement, a spokesman for Bewl Water said: “On Monday, July 19, emergency services had to be called after a woman was struck by lightning at Bewl Water during a thunderstorm at approximately 4.15pm.
“The team at Bewl Water performed CPR until paramedics arrived on the scene.
“The injured woman was stabilised before being transferred to a London hospital by road.
“Our thoughts are with the family during this very difficult time.”
One of the attending paramedics later thanked the team at the reservoir.
He said: “We’d like to pass on our thanks for what the Bewl team did in a very difficult situation.
“Their response made our job easier, and thanks to their efforts the lady could be moved to London and has been stabilised. The Bewl team should be immensely proud of their efforts.”
A spokesperson for South East Ambulance Service added: “The woman was given emergency treatment at the scene before being taken to King’s College Hospital in a serious condition, with the air ambulance team travelling in the ambulance to help treat the patient en route.
“We would like to thank all those who came to the woman’s aid prior to our arrival.”