The incident happened at around 3.40pm on Monday, January 17, on the B780 at its junction with the B784 near Dalry.
The crash involved a white Vauxhall Insignia and a red Kia Venga.
An 89-year-old man, the driver of the Kia, was taken to the Royal Alexandria Hospital in Paisley following the crash.
However, he sadly later died on Sunday, January 23.
The pensioner’s family is aware and a report has been submitted to the Procurator Fiscal.
Police Scotland is appealing for information after the incident.
It is understood that some people stopped to assist at the scene and may have witnessed the crash taking place.
The police are asking these people to come forward to assist with their enquiries into the incident.
Sergeant Chris Hoggans, of the Ayrshire road policing team, said: “Our thoughts are very much with the man’s family at this difficult time for them.
“We are continuing to investigate the circumstances surrounding the crash and would ask anyone who has not yet come forward with information to get in touch.
“From our enquiries so far, we are aware that some people stopped to assist and may have witnessed the crash so I am asking them to come forward. We are also keen to hear from anyone who may have dash-cam footage from around the time of the crash.“Anyone with information is asked to please contact us on 101 quoting incident 1970 of Sunday, 17 January.”