Residents in villages such as Lamphey, Milton and Cresselly remember US troops organising dances and giving sweets to children as they prepared to fight.
The soldiers then joined the Battle of Normandy, which was key to bringing the Allies victory in World War Two.
The new memorial was unveiled at the Carew Cheriton Control Tower.
This year marks the 75th anniversary of the 1944 D-Day Landings, when the 110th Regiment, which was based in Pembrokeshire, suffered major losses.
Only 532 of its 5,000 soldiers were fit for duty at the end of the war after they had advanced through France and Belgium.
John Brock, who was 15 in 1944, said: “They used to come through here on their Sherman tanks.
“We’d wait for them in Milton and as they’d go through they’d throw gum and Lucky Strike cigarettes to us.”
The memorial would help local residents to “remember the cost of our freedom”, Mr Brock added.
The soldiers were based in Pembrokeshire from August 1943, rehearsing for D-Day on nearby beaches such as Saundersfoot.
In 1944, the leader of the 110th Regiment wrote to the mayor of Haverfordwest to apologise for his troops’ “unceremonious departure” and thanked people for their hospitality.
Now there is a permanent memorial remembering their contribution.
D-Day veteran Ted Owens, 94, from Pembroke Dock, was wounded in the battle.
“If it wasn’t for the Americans, we wouldn’t have won the war,” he said.
“I met a lot of them in Pembroke Dock and I can still remember their names and where they were from after all these years.”