The 21-year old will be reunited with Wales international brother Aaron at the region, who has signed a three-year extension with the region.
Steven Shingler says he's delighted to have the opportunity to return back to Wales and play his rugby in alongside brother Aaron.
Steven Shingler, who has appeared 39 times for London Irish to date said: "Although I left the region, I've always remained a supporter and the Scarlets will always be a huge part of my life. You never lose that if you've grown up supporting Llanelli as a youngster - it's a strong rugby family and one of most passionate rugby communities around.
"I've had some great experience from my time at London Irish and it's been good for me to play in the intensity and physicality of the Premiership.
"During my time at London Irish, I've been lucky to work with and learn from some great people like Mike Catt who have pushed me forward and improved my game. My aim before leaving Llanelli was to get experience and plenty of rugby and I hope to achieve 50 appearances for London Irish and that can only be great experience for me going forward.
"I've been following the Scarlets performances throughout and there's a very talented squad of players that I'm excited to be a part of. I look forward to working with the coaching set-up and I know from my brother how committed, close-knit and hard working the group at Parc y Scarlets is."
The utility back says the eligibility tug of war he was engulfed in last season between Wales and Scotland was well behind him and he just wanted "to move on".
Aaron Shingler, who has four Wales caps to date and 57 appearances for the Scarlets says he was looking forward to being reunited on the rugby field with his brother at Parc y Scarlets.
"I am committed to the Scarlets and delighted to have secured an extension to my contract here at Parc y Scarlets for the next three seasons. There's a lot to come from our group and we know and believe as players we have the potential to achieve much more going forward.
"I've always enjoyed my rugby here and the chance it gives me to develop at international level as well. I'm grateful for the opportunities and support I have been given by the region in pushing forward my rugby career and want to give as much I can in the future.
Scarlets Head Coach Simon Easterby said: "It's important for our progression to have secured these two talented players from our region who will add real value to our squad going forward. Steven will fit into our Scarlets system very well as he's grown up in our region - he's a versatile and ambitious player who has broadened his horizons and benefited from the two seasons with London Irish.
"Aaron is a committed professional, his career is on the rise and it's been good to see his quick progress at international level as well as with us at Parc Y Scarlets. He's a strong, athletic and intelligent player and will be an important to us as a senior figure in our pack as he gains more experience at the top level of the game."