The 22-year-old joins with an obligation to buy next summer and the deal will see Phillips contracted to the club until summer 2026 with the club holding an option to extend for a further year.
Phillips has had a number of loan spells with Shrewsbury Town, Wycombe Wanderers as well as experience in the SPFL with Aberdeen.
Speaking to stmirren.com the youngster cannot wait to get started.
"I spoke to the gaffer and he sold the club to me. He told me about all the players that have come through St Mirren and how he likes to develop players.
"I'm a box-to-box midfielder and I'll always give 100%. It's been a long summer, but I'm delighted this deal is done and can't wait to get going."
Manager Stephen Robinson has also expressed his delight in signing the young star.
"Killian is somebody we have looked at for a while.
"He’ll improve with the coaching, but he has the physical attributes. He’s a big boy, he can run, and he’ll hopefully help provide goals in the team as well. With a couple of bumps and knocks in the midfield it’s important we strengthen our numbers in there."