"The shoulder is great. I could have played last week but I wanted that extra week of rehab," Mortlock said.
"There's a lot of hype about this match and I'm just excited to be playing again."
Australia coach John Connolly has made only one change to the side he named against Wales.
Blockbusting winger Adam Ashley-Cooper, who missed that game with a toe injury, comes into the side on the left wing for Drew Mitchell, who drops back to the bench despite being the tournament's leading tryscorer with seven.
The versatile Julian Huxley is the only other back on a bench loaded with forwards, highlighting Australia's intentions to match England's massive pack.
"Most of our backs are fairly multi-skilled so we're confident about our lineup," Connolly said.
"We've gone down that path in most of our games this year".
Despite the Australians being favourites to win, Connolly remains wary of England's forwards and the deadly kicking of their flyhalf Jonny Wlikinson.
"England is a very dangerous team," Connolly said.
"Their set piece has been their cornerstone for many years and they have incredible speed out wide and a number 10 who steers them around exceptionally well.
"When you look at our results through the pool games you'd have to say we're pretty happy with where we're at.
"But we're very conscious that we have to improve considerably on that to go any further in the tournament."
Meanwhile, Andy Farrell has pulled out of England's lineup with a calf muscle injury.
The rugby league convert had been recalled to play at inside center against the Wallabies but England announced on Thursday his right calf suffered a reaction after training on Wednesday.
Farrell had a clinical review and MRI scan on Thursday which confirmed a strain and the Saracens center is now getting treatment.
England coach Brian Ashton will not announce who replaces Farrell until Friday's news conference.
The likely replacement is Olly Barkley, who was left out of the starting lineup to make way for Farrell's return.
Australia team: 15-Chris Latham, 14-Adam Ashley-Cooper, 13-Stirling Mortlock (captain), 12-Matt Giteau, 11-Lote Tuqiri, 10-Berrick Barnes, 9-George Gregan, 8-Wycliff Palu, 7-George Smith, 6-Rocky Elsom, 5-Dan Vickerman, 4-Nathan Sharpe, 3-Guy Shepherdson, 2-Stephen Moore, 1-Matt Dunning.
Replacements: 16-Adam Freier, 17-Al Baxter, 18-Hugh McMeniman, 19-Stephen Hoiles, 20-Phil Waugh, 21-Julian Huxley, 22-Drew Mitchell