Thirty firefighters and four appliances have been at the Co Down peaks since first light on Sunday.
Aidan Jennings, assistant chief fire and rescue officer, said the operation will continue across the day.
An update on the Mourne Mountains wildfire, from @NIFRSJennings.
There are currently 6 Fire Appliances & 40 Firefighters in attendance.
ACFRO Jennings thanks our NIFRS personnel, partner agencies and the businesses & individuals who've provided refreshments, for their support. pic.twitter.com/F1Q9fWKMZn
Firefighters efforts have been supported by police, Coastguard, Mourne Rescue Team, Forestry Service, National Trust, NIEA and Sky Watch Patrol.
Coastguard helicopters from the Republic of Ireland and Great Britain allowed fire chiefs an aerial view of the blaze to help inform tactics, transport personnel to remote locations and plan resources.