The major roadworks between Halfway and Llandovery will run from October 12 until May 6.
The project includes repairing a landslide-prone slope, installing a new drainage system, and adding safety barriers.
To minimise disruption, the A40 will have limited weekend or evening closures, with a single lane closed on weekdays, managed by temporary traffic signals.
A temporary 40mph speed limit will be enforced throughout the works.
The westbound lane will remain closed outside of road closure periods to ensure safety due to temporary slope instability.
This measure will be reviewed during the project, and the lane will be reopened whenever feasible.
The Welsh Government has stressed the importance of the work for the road’s long-term viability.
A spokesperson said: “We understand this will cause disruption in the short term while the work is underway, and we thank drivers for their patience.
“It is vital this work is carried out at this time for the future resilience of the road.”
Motorists are urged to plan their journeys in advance, considering potential delays.
The works are expected to maintain the road’s safety and stability for years to come.
