Emergency services were called to the collision between a Nissan Qashqai and Vauxhall Corsa on the A308 Marlow Road near Maidenhead on Wednesday.
The driver of the Vauxhall - a 38-year-old woman - was taken to hospital with life-threatening injuries and died in hospital on Monday.
A nine-year-old girl in the same car was taken to hospital with serious injuries and has since been discharged.
The three occupants of the Nissan all sustained minor injuries.
No arrests have been made in connection with the crash.

Eilish Herron, of Paisley, was fuelling her Renault Clio at the forecourt in Linwood, at about 14:30 on Wednesday, when it was hit by a Ford Focus.
The 17-year-old died at the scene. The male Ford car driver was uninjured.
Eilish's former school, St Andrew's Academy, have also said she was "very popular" and "a wonderful young adult".
In a statement, Eilish's family said: "We are absolutely devastated by the loss of our beautiful daughter Eilish.
"We just cannot believe that we will not see her again. She was a lovely girl, a good daughter, sister, granddaughter and a great friend to those who knew her. She had so much to live for and we will miss her terribly.
"We are very gratefully for the thoughts and kind words already given, however, we would ask that we be allowed to have the time to grieve as a family at this very, very sad and difficult time."
Kevin Henry, head teacher at Eilish's former school in Paisley, said: "Eilish enjoyed six very successful and happy years in St Andrew's Academy and had left at the summer. She had been accepted to study nursing at Robert Gordon University.
"Eilish was a wonderful young adult and was very popular with both her peers and the staff of the school.
"She was a talented sportswoman and had been recognised in 2013 as Renfrewshire Young Sportsperson of the Year."
Mr Henry said Eilish had given "much of her own time to coaching younger students in the school".
"Eilish was a great role model for our pupils, her death is a tragic loss," he said.
"Staff and the school chaplain will be available to students who require any support. Eilish and her family will be in the thoughts and prayers of all in the school."
Meanwhile, police have asked anyone with information about the incident in which Eilish died to contact them.
A report on the circumstances will be submitted to the procurator fiscal.
A woman has died from her injuries five days after a car crash in Berkshire.
[NEXT_CONCEPT]
The family of a Renfrewshire teenager who died after a collision between two cars at an Asda petrol station have said they are "devastated" by her loss.