The former club captain replaces James McFadden as the No.2 to Stephen Robinson, and has decided to concentrate fully on his new role by calling an end to his lengthy playing career.
The 37-year-old made close to 500 appearances for the Fir Park club across two spells, for which he was rewarded with a testimonial match in 2015.
Lasley says the move into coaching is one he’s been anticipating for quite some time, and he’s looking forward to the next chapter of his life.
He told Motherwell’s website: “It’s a big change obviously, but this is something I’ve been planning for a long time and this side of the game is something I’m passionate about.
“It’s not fully sank in that I won’t be pulling on that jersey again but I’m sure that will take a little bit of time – but the excitement of this opportunity is starting to outweigh that certainly.
“I’m just thankful to have been around this place for as long as I have – from the first day I walked in here, off a building site in to Motherwell and thinking that I would go on to play however many games, captain the club, play in Europe and play until the age of 37 is beyond my wildest expectations.”
Robinson added: “I spent a great deal of time speaking to a number of potential people for the post over the last few weeks, but Keith was by far and away the outstanding candidate for the job.
“He is extremely qualified in that he has his UEFA B, his A and his Pro Licence and has coached with our Under 17s and our Under 20s, but just as important, it helps keep a ‘Motherwell’ identity within the coaching staff, which I personally think is extremely important.
“He understands what it is like to play and do very well here, and has a huge passion for Motherwell FC and the supporters. I am really looking forward to working with him.”