The 46-year-old replaces Kurt McQuilkin, who left for family reasons, and will work under head coach Leo Cullen.
"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," said Leinster chief executive Mick Dawson.
"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.
Since leaving England, Lancaster has had short-term stints with the Atlanta Falcons American football team, British Cycling and the English Football Association before he returned to rugby union with Counties Manukau in the Mitre 10 Cup in New Zealand.
(Reporting by Neil Robinson; editing by Amlan Chakraborty)
