Lester arrives from Sheffield United, having also worked as assistant coach with Wales. Lester worked with Bees manager Keith Andrews at the Blades.
Lester joins Brentford after Martin Dury's sudden departure last week.
“This is a position we’ve been keen to fill for some time,” said Andrews.
“We’ve discussed the role with Jack during these last few weeks and, as a club, we feel that he is the right person for this job.
“Individual player development is an area we really focus on. We are a club that helps players improve and get to their very highest level, it is part of our ideology. We want Jack to bring this area together and build on the impressive work done by Steven Pressley over the past four years.
“Jack and I worked together at Sheffield United and got on really well - I’ve seen first-hand how he can develop players, and how passionate he is about developing players. His values tie in with what we are as a football club.”