The Australian midfielder endured an injury-plagued spell at Oakwell, making just six appearance in Barnsley’s promotion-winning campaign of 2015-2016.
The 24-year-old admits to feeling “on the outside” as he watched the Reds beat Millwall in the League One play-off final thanks to goals from Ashley Fletcher, Adam Hammill and Lloyd Isgrove.
In an ironic twist of fate Rotherham are hoping to take Barnsley’s place in the Championship – the Tykes were relegated at the weekend – as they prepare to face Scunthorpe United in a two-legged play-off semi-final.
“When I was at Barnsley and we got promoted I was injured at the time,” Williams told The Yorkshire Post.
“So, unfortunately I missed out on that, so I am really excited. The chance to play at Wembley and get back to the Championship. It’s a place where this club belongs, the stadium, fans, staff, players. It’s where we need to be.
“I felt on the outside at Barnsley, 100 per cent. For me it was like being on the outside looking inwards. But it was good for my career, in a way. It was bad I missed a lot of football, but for me personally I learned a lot.
“Looking for a silver lining, I try to take the positives from it, which I think I have done.”
Since moving across South Yorkshire last summer, Williams – who started his career in England with Portsmouth before a move to Fulham – has been injury-free with the Millers.
He has featured in 42 out of 46 league matches and has been a key player for Paul Warne’s team.
“It’s been a good season considering my past injuries,” said Williams. “I have been available for every single game, played 42 games, only missed out on four.
“For me it’s been a success. There was bound to be a little dip in form for me, around Christmas time, and I went out of the team.
“That was to be expected because of the small amount of games over the last two years.
“But I am happy to have got through the season and feel fresh. Hopefully there’s another three games.
“When I signed here I felt this was the best platform to get to the Championship.
“Personally my goal was to play as many games as I could; the other goal was to be available for as many games as I could.
“I managed to start 32 games, which for me is big. I don’t think I missed a training session, which is awesome, and the injury is way behind me. I am really happy with the season.”
Rotherham travel to Scunthorpe for the first leg on Saturday before the teams reconvene at the New York Stadium next Wednesday.
The Millers have completed a league double over the Iron this season. Goals from Will Vaulks and Kieffer Moore gave Rotherham a 2-0 home win in October and the South Yorkshire side enjoyed a 2-1 success at Glanford Park in February, with Joe Newell and Semi Ajayi on target.
But Williams does not want to focus on previous results.
“We need to forget that, put that to bed,” he said. “They have been in good form, but it doesn’t scare us. Nobody scares us. I think we are the better team.
“A plus is we don’t have to have an overnight stay, or far to travel. It’s good for us because for the fans it’s just down the road. We need to go there, win, then come home and run all over them in the second game and get to Wembley.
“The fans deserve that because they have stuck with us all for the whole season.
“With the boys we have got I can’t see us losing. Everyone will give that little bit extra, we have that little bit extra juice in the tank.”
Of the four teams in the League One play-off positions, only fourth-placed Rotherham won their final game of the regular season last weekend.
Vaulks netted the goal against Blackpool in a 1-0 win while Scunthorpe were held 1-1 at home by Bradford City.
Third-placed Shrewsbury Town lost 1-0 to MK Dons and Charlton lost at Rochdale 1-0.
On the Blackpool game, Williams reflected: “We had a good performance for about 75 minutes, but then gave them too many chances.
“To be fair, if they had their shooting boots on it probably would have been a different story, and we wouldn’t have been as happy as we are going into the play-offs – but it was nice to get three points.
“It can be a little dodgy at times because it didn’t really mean too much.
“I’m just glad we came out on top – even if we had lost, it wouldn’t have made much difference. Everyone would have stepped up in training ahead of the play-offs. But we are the only team who won (on Saturday) out of the play-offs so we will take any advantage we can get.
“This week will be a normal week, just keep doing exactly what we have been doing, it’s working. I don’t think the gaffer will change much.”