The center made an official visit to Saint Louis two weeks ago.
Manning averaged 10 points, 8 rebounds and 3 blocked shots per game as a junior as Chantilly went 21-4 and advanced to the state semifinals.
He averaged 13 points, 10 rebounds and four blocked shots per game during his sophomore season.
Manning is SLU's first commitment in the Class of 2011 and he fills a big need inside for the Billikens.
If everyone from the 2010-11 roster returns for the 2011-12 season, Manning's first year on campus, SLU's 6-9 Willie Reed would be a senior, 6-8 forward Cory Remekun would be a junior, 6-11 forward Rob Loe would be a sophomore.