Williams had eight rebounds, Mitidiero scored 22 points and Allie Quigley had 12 points and six assists for the Blue Demons (3-0).
Mara Freshour had 14 points to lead the Seminoles (2-1), who hit just 10-of-30 shots in the second half after shooting 53.8 percent in the first half. Freshman Courtney Ward added 12 points.
DePaul limited Florida State's leading scorer Jacinta Monroe (18 points per game coming in) to eight points on 3-for-8 shooting.
DePaul opened the second half on a 21-7 run, taking a 59-44 lead on China Threatt's 3-pointer with 12:32 left in the game.
A 10-5 run by Florida State cut DePaul's lead to 66-57, but the Blue Demons countered with 11 straight points to put the game away.
Florida State looked sharp early as they took a 21-12 lead on a layup by Antionette Howard with 7:15 left in the first half.
DePaul used an 8-2 run to draw to two points and then took its first lead of the game, 32-31, on a 3-pointer by Mitidiero with 1:44 left in the half.
Mitidiero had 16 points, including four 3-pointers, in the first half as DePaul held a 38-37 lead at halftime.