Neil Jukes' side dropped to the foot of the Betfred Super League table following Thursday's 36-10 defeat at Widnes Vikings and the Centurions will hope that Mortimer can bring an upturn in results after he agreed a deal until the end of the 2019 season.
"Daniel is another proven quality NRL player with lots of experience at the highest level, including Grand Finals," said Leigh owner Derek Beaumont.
"He is a running pivot with a great kicking game and can also run at nine so is a very versatile player.
"Again it is all money but it is not only about ensuring we maintain our Super League status, it is also about improving the squad for next year as I want this club to become more and more competitive so that over time we can win something."
CENTURIONS CONFIRM THE SIGNING OF DANIEL MORTIMER
— Leigh Centurions (@LeighCenturions) June 25, 2017
For the full story please see https://t.co/Xx4q8uyClb pic.twitter.com/6IVIPHdayS
The 28-year-old made his debut for NRL champions Cronulla only last weekend and featured again on Sunday but the Sharks are happy for Mortimer to join Leigh immediately.
"I'm really looking forward to this next chapter in my career," said Mortimer, who has also played for Parramatta Eels, Sydney Roosters and Gold Coast Titans.
"I have been following Leigh closely now for the past six weeks and I hope I can play a significant role for the club between now and season's end.
"Playing overseas has always been an attraction and I am lucky that a great club like Leigh took the initiative to recruit me.
"It will be challenging but I'm always up for a challenge and hopefully we will do well in the weeks ahead."
SAM HOPKINS SIGNS NEW THREE-YEAR DEAL WITH LEIGH CENTURIONS
— Leigh Centurions (@LeighCenturions) June 25, 2017
For the full story please see https://t.co/Xx4q8uyClb pic.twitter.com/SldgauqBTR
Leigh also announced prop Sam Hopkins has signed a new three-year contract.
The 27-year-old is now tied to the club until November 2020.
Hopkins told the club's website: "I feel as though I am approaching my peak years as a player and I now want to crack on with realising my potential and not giving myself any excuses for not succeeding at being at the top of my game."
