Goals from Ryan Dodd and Ben Trafford helped them to a 2-1 win over their fellow West Lancashire League side.
Dodd scored from close range early on and Fulwood missed some good chances as the first half went on.
The second half was an even affair, Trafford rising to head home at the far post to double the lead.
But Slyne finished strongly, halving the deficit after a long throw-in had caused problems in the box.
It led to a tight finish but Fulwood held out to clinch victory.
A healthy sized crowd of 460 watched the game the LFA’s County Ground headquarters.
Fulwood manager Tony Hesketh said: “It wasn’t a vintage performance but it was a cup final victory as I told the lads afterwards.
"I thought it was a good game, both sides played extremely well and were dogged.
"After taking the lead we missed a few chances and in those circumstances you do fear they will come back to bite you.
"Fortunately we got the second goal to settle us a bit more but every credit to Slyne, they gave it a right go.
"For a spell they were the stronger team and got a goal back.
"That put us under pressure but I thought we saw out the closing stages quite well.”
It was the second time in four seasons Fulwood have won the Amateur Shield.
Hesketh said: “FC St Helens were the holders, there was a year when it wasn’t played because of Covid, then it was us again before that.
"The support we got in the final was brilliant, we have 13 teams associated to us and all were represented there. It was great to see so many people.
"We had a new kit for the night provided by our sponsors Nugent Santé. We thank them for that and all their support.
"We’ve got a promising young team, I think only two of Tuesday night’s team started the last final we played in so that shows how many younger players we have introduced.
"As a club we are thrilled to win the trophy and hopefully it will give us a lift in our tilt at the league title.”