The 6-foot-5 guard initially signed with Randy Bennett and the Gaels out of high school, but after his father, Craig, was promoted to head coach at New Mexico, the younger Neal decided to play for his dad.
"It's like I've come full circle," Cullen Neal told ESPN. "I was comfortable with them coming out of high school and took a visit there this past weekend and still felt comfortable."
"Saint Mary's has a winning culture," he added. "They have a really good team coming back and I just want to help in any way I can. The biggest thing for me was to go out as a winner."
Cullen Neal left after three seasons and played last season at Ole Miss -- where he averaged 9.4 points per game. He received a medical redshirt for his sophomore season due to the fact he missed most of the year.
Saint Mary's finished 29-5 last season and returns four starters from last year's team.