The former world player of the year starts on the wing for his hometown club in Sunday’s National Bowl final against Caerphilly, joining both his brother and brother-in-law in the back three.
And Mangan, who led his team to a 29-3 semi-final win against Cardigan, is expecting Williams’ know-how to make the difference on the day.
He said: “We cannot deny Shane’s experience and the confidence he is going to give us.
“Not every club can boast a British & Irish Lion on the wing for them. He will talk us through the day emotionally and ground us a little bit if we are too excited.
“He’ll help keep everybody’s heads down. He has been there and done it.”
Mangan is relishing the chance of leading his team out at the home of Welsh rugby when the Bowl kicks off a day of amateur rugby action.
“It is something you dream of [leading the team out],” Mangan added.
“When you know you are not going to make it as a professional, playing for your village club is the next best thing. Walking out with your mates with everyone watching will be great.
“Everybody says it is the best stadium in the world so that makes it even better.”
Amman beat last season’s Bowl winners Burry Port on the road to the final as well as overcoming Hartridge, Monmouth and St Albans.
Caerphilly captain Gareth ‘Bob’ England is looking forward to achieving a “boyhood dream” when he leads his team out on the Cardiff turf.
“I am definitely excited,” he said.
“It will be a first experience for me here. It is a boyhood dream as a kid, running out here at this stadium for your home club, I could not ask for anything more.
“We have a few buses coming down here Sunday. It will be great for the club and the town.”
England reckons it is vital his team do not solely focus on stopping 87-cap former Wales wing Williams.
He added: “Shane was a world player of the year, and it’s exciting to play against him.
“But you cannot base your plan around one player, there are 14 other players on the field. You have to take it as it comes.”
The Cheesemen – second in East Division 3A – are on course for a League and Bowl double having thumped Llanhilleth 61-7 on Tuesday.
It helped close the gap to table-toppers Hartridge to four points with two games in hand.
However key player Will Oliver dislocated his shoulder in that match and will miss Sunday’s game.
Caerphilly reached the final after beating Cilfynydd 20-14 in the semi-finals.
Matthew Hurley and Josh Hayward crossed for tries with the former adding another 10 points from the tee.
Caerphilly: Tom Candy; Lewys Willacott, Matthew Nuthall, Chris Thomas, Gareth Dare; Matthew Hurley, Jac Jonathan; Stuart Williams, Rhys Cleverly, Ben Sparey, Gareth England (capt), Gavin Bilton, Chris Moss, Will Oliver, Josh Williams
Replacements: Kieran Thomas, Gareth Ogden, Luke Pearson, Damien Corns, Tom Azenderrie, Josh Hayward, Joe Bray, Connor Kawulock
Amman United: Gavin Lewis; Dean Williams, Talfan Alcock, Mathew Brayley, Shane Williams; Rhys Thomas, Joel Foster; Tom Davies, Doug Jenkins, Liam Mangan, Chris Rees, Shaun Cadell, Alun Lewis, Dan Jones, Sean Mangan
Replacements: Gavin Madge, Liam Danter, Steff Davies, Scott Stephens, Rhys Phillips, Robert James, Kiearan McKay, Tom Thorneycroft
