A number of Australia's players had been warned their places in the side were not nailed down, with the use of 17 players throughout the series proving such a point.
But with England having already wrapped up the series as they led 3-0 going into the match at the Kia Oval, both Smith and Shane Watson took the opportunity to stake their own personal claims for starting berths when the next Ashes series begins in the winter.
Watson was the star on the opening day but Smith took the lead on day two, which was affected by rain with no play possible until 2:30pm.
The 24-year-old played some classy shots as the ball swung more than it had on day one, with Smith admitting he was pleased to score his first 100 at the right time.
"I probably was under a bit of pressure," he said.
"But I was trying to put it in the back of my mind and play the way I always play.
"I didn't think too much about it and just tried to go out and get runs and it was nice to form a partnership with Watto [Watson] yesterday, he batted superbly and was able to put us in a good position."
Smith reached three figures by smashing a six off the bowling of Jonathan Trott, who had been given the ball by captain Alastair Cook despite the fact his side boasted a five-strong bowling attack.
To bring up his maiden century in such a fashion will make it all the more memorable for Smith, who revealed afterwards he has already planned to target Trott before delivering the stylish six.
"I probably wouldn't have played quite the same shot if it was someone else bowling," he said.
"Jonathan Trott is obviously a part-timer who doesn't bowl too often and I said to Hadds [Brad Haddin] between overs about hitting him over his head and he just told me to keep a clear mind and if it is there go for it. It was there and I managed to get it away.
"It wasn't Jimmy Anderson or Stuart Broad so it was a good chance I saw to get it out of the way and it came off in the end."
Smith's best before Thursday's unbeaten 138 was 92 in Mohali earlier this year.
He also struck 89 in the drawn Ashes Test at Old Trafford before being caught off the bowling of Graeme Swann but always felt the first century was within his grasp after working his way back in to the fold.
"I didn't think it was far away," he added.
"Obviously it was disappointing to get out for 89 at Old Trafford to a pretty silly shot but I feel like I have been hitting the ball really well and I guess my patience has been pretty good. I hoped it wasn't too far off and today was the day.
"I think that some chances definitely slipped - I got a couple of 50s when I was going pretty well and those are the ones you have to try and convert and make big scores, those are the ones that win the games for your team.
"I feel like I'm improving, my mindset is good and I'm getting better mentally so it is all about improving and just trying to become a better player every day."