It happened at Pinewood Avenue in Woodburn. The man is understood to be a high-profile loyalist.
The ambulance service received a call about 14:15 GMT following reports of a man with a gunshot wound.
After treatment at the scene by paramedics, he was taken to the Royal Victoria Hospital in Belfast.
In the aftermath of the attack, armed police wearing protective masks carried out raids on property in the area.
There has been an ongoing feud between loyalist factions in Carrickfergus for some time.
It is understood one line of enquiry is that the shooting could be connected.
PSNI Supt Darrin Jones said that while the investigation was "still at an early stage police are following significant lines of enquiry".
"Firearms being discharged in a residential area, particularly during daylight hours when children are in the vicinity, is concerning," he added.
"We need the community to give us the information needed so we can remove the dangerous individuals who carried out this attack from our streets and put them before a court of law.
"There will be a continued police presence in the area as we search for those who carried out this brutal attack."
Alliance MLA for East Antrim Stewart Dickson said a "shooting is a very violent incident and it will undoubtedly have caused a great deal of fear and concern".
Ulster Unionist MLA John Stewart said he was "shocked" to hear of the attack and that there was "no justification for such actions".
Detectives are appealing for anyone with information to contact them on 101 or Crimestoppers anonymously by telephoning 0800 555 111.
A man in his 40s has been taken to hospital with life-threatening injuries after being shot in the neck in Carrickfergus, County Antrim.