McCombs rushed for 94 yards and his fourth-quarter touchdown. McCummings, who also came in at quarterback occasionally, ran for a 3-yard score that gave the Huskies a 14-0 first-half lead.
That came after Williams scored the game's first touchdown on a 58-yard punt return.
Connecticut (2-1) started quickly in going against their former coach, Randy Edsall, who left after 2010 to come to Maryland (2-1). Both sides spent much of the week downplaying talk of any hard feelings.
The Huskies outgained Maryland 148-80 in total yardage in the first half and controlled the ball for 18 minutes, 40 seconds. They also came up with timely plays, converting four of nine third-down situations to keep the Maryland offense off the field.
Connecticut opened the scoring on Williams' 58-yard punt return, and Chad Christen's extra point gave the Huskies a 7-0 lead with 7:05 left in the first quarter.
They took advantage of a short field to score their second touchdown, gaining possession at the Maryland 36 after the Terrapins were forced to punt from their 4. McCummings' 3-yard run seven plays later made it 14-0 with 12:35 left in the half.
Maryland's offense finally awakened on the next possession. A key play came on fourth-and-4 from the Connecticut 37, when quarterback Perry Hills completed an 18-yard pass to Stefon Diggs.
One play later, Wes Brown ran for a 19-yard touchdown, and Brad Craddock's extra point cut the lead to 14-7 with 10:24 remaining in the half.
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Connecticut linebacker Yawin Smallwood sacked Hills and forced a third-quarter fumble that Angelo Pruitt recovered for the Huskies at the Maryland 23. They couldn't pick up a first down, but Christen kicked a 34-yard field goal with 3:29 left in the quarter for a 17-7 lead.
Smallwood caused problems for the Terrapins throughout the game, with four of his 14 tackles going for losses, including three sacks.
After a 75-yard drive, Maryland made it 17-14 on the second play of the fourth quarter when Hills threw a 29-yard touchdown pass to Diggs on third-and-10. The Terrapins' Marcus Leak tipped the pass and Diggs grabbed it for a touchdown to make it 17-14.
But the Huskies answered on their next possession, driving 76 yards to take a 24-14 lead on McCombs' 11-yard touchdown run.
Hills ran for a 10-yard touchdown that cut the Connecticut lead to 24-21 with 4:39 left.
After getting the ball back following a Huskies punt, Maryland reached the Connecticut 39 before its drive stalled with 17 seconds left.