The 46-year-old has been named as a senior coach following the departure of defence coach Kurt McQuilkin.
Lancaster spent four years in charge of the national team before resigning from his post after the Red Rose's failure to progress past the pool stage at their home World Cup last year.
Since stepping down, Lancaster has had spells with the Atlanta Falcons, British Cycling, the English Football Association and New Zealand side Counties Manukau.
Head coach Leo Cullen told Leinster's official website: "The sad departure of Kurt McQuilkin due to family reasons has brought about an opportunity to bring in somebody new to the Leinster Rugby coaching team.
"Stuart brings a wealth of experience and is a highly respected figure in world rugby. We are all looking forward to having Stuart join our team and together helping the players maximise their potential as a group."
Chief executive Mick Dawson added: "We are always looking at options around resources but once we were made aware of Kurt McQuilkin's departure, we narrowed our focus and we are delighted that Stuart has agreed to come on board.
"I know, having spoken at length to Stuart, that he is excited by the challenge of getting into club rugby again and the day to day interaction with players. His first session with Leo, Girvan, John and the players will be later on this afternoon.
"We welcome Stuart to the club. I think our supporters are going to be excited by this announcement and we wish him all the very best for the season ahead."
Before taking on this role, Lancaster reportedly held talks with Toulon regarding a possible switch to the Top 14 club.