Mike McCall Jr. had 10 points and seven rebounds for the Billikens (23-6, 11-4 Atlantic 10), who moved a half-game behind first-place Temple in the league standings. The Owls, however, hold the tiebreaker, a 72-67 victory against St. Louis in January.
The Billikens clinched a top-four seed and bye in next week's Atlantic 10 tournament.
Xavier led 39-29 at halftime but shot just 28 percent (7 for 25) afterward. That helped St. Louis take control with a 24-2 run over an 8:34 second-half stretch.
Conklin had 11 points and McCall hit two 3-pointers during the surge, which gave the Billikens a 53-42 lead with 9:44 remaining. The Musketeers got no closer than seven points the rest of the way.
Maryland point guard Melo Trimble, considered a lock to leave the program before struggling in the second half last season, has withdrawn from the NBA draft and will return for his junior season.
Louisville's Chinanu Onuaku, who recently underwent a medical procedure to correct a heart rhythm issue detected during the combine, will keep his name in the NBA draft pool.
Andrew White III will return to the Nebraska men's basketball team next season after removing his name from the NBA draft.