Hill, the Big Ten's leading scorer, broke the 76-all tie with a three-pointer at the 4:56 mark, and Prahalis followed with a triple of her own 49 seconds later.
After Illinois point guard Lydia McCully made one of two free throws, Prahalis hit another three-pointer then answered a basket by Illinois forward Karisma Penn with a jumper of her own to make it 87-79.
The two teams traded single free throws, then Ohio State pushed the lead back to 10 when Hill took the ball away in the lane and threw it ahead to Prahalis for a fast-break layup with 2:06 to go.
"I don't think it should have gotten to that point but it was just like we wrote up a play, Tay hit a three," Prahalis said. "We had out of bounds, Coach (Jim) Foster wrote up a play, I hit a three and we just kept rolling."
The Fighting Illini (6-14, 0-7) would not get closer than eight in the final two minutes. The third of Ohio State's veteran guard trio, Amber Stokes, made four of four free throws to salt the game away and allow the Buckeyes to improve to 19-1 overall and 6-1 in Big Ten play with a 96-84 victory.
Prahalis finished with a game-high 28 points, and Hill came in right behind her with 26. Stokes contributed a career-high 16 while sophomore forward Darryce Moore added 10 points and four rebounds before fouling out in front of a season-high crowd of 6,687 at Value City Arena.
"I think the team knows and we know as veteran guards, all of us including Amber, that we just need to get a good shot," Hill said of the mindset when things tightened up late.
Of her go-ahead three, Hill explained, "We reversed the ball and Amber attacked and my girl helped a little bit and she kicked it out."
Penn had 23 points while guards Adrienne Goldbold, Amber Moore and Ivory Crawford added 18, 13 and 12, respectively, for Illinois.
The Illini's losing streak hit seven games, but this was the first one by more than seven points since a 70-50 loss to Michigan on Dec. 30.
"We're still positive," said Penn, a junior from Shaker Heights, Ohio. "We could easily be 5-2, so we know it's bound to happen. We've just got to keep working hard. We're in all these games. It just kind of gets away from us at the end."
Ohio State shot 56.9 percent for the game, including 69.6 percent (16 for 23) in the second half.
"I think my kids played composed, they didn't give up or get rattled," Illinois head coach Jolette Law said. "I think we played pretty good but down the stretch we gave up a couple of backdoor cuts, had some miscommunication, but I think we're getting better daily. I try not to look at the record.
"I give a lot of credit to Ohio State, especially Sammy Prahalis and Tayler Hill. Big-time players make big-time shots. Down the stretch Sammy put this team on her back and made some big plays."