Loosehead Barrington, 26, has featured 82 times since joining Sarries from Jersey in 2013, and has agreed a three-year contract at Allianz Park.
USA international Lamositele, 21, has signed a new two-year deal.
"Both Richard and Titi are valuable members of our squad," director of rugby Mark McCall said.
Prop forwards Richard Barrington and Titi Lamositele have signed new deals with Premiership and European Champions Cup winners Saracens.