We describe the design and implementation of an undergraduate peer mentor program that can overlay an existing general chemistry laboratory and is designed to improve STEM student retention. For the first four freshman cohorts going through the program, year-to-year retention improved by a four-year average of 20% for students in peer-mentored laboratories versus those not in peer-mentored laboratories. A majority of students in peer-mentored laboratories recommend continuation of the program, 85.4% of 765 students in the first three cohorts. Our program is uncomplicated and requires only modest financial resources to affect significant increases in STEM retention.