Illinois has lost five of seven -- and three straight in the Big Ten -- after starting 12-0.
The Wildcats (11-7, 2/3 Big Ten) were 8 of 15 from 3-point range but hit five of those shots in the first half to build a 36-21 lead.
The Cats were coolly efficient, early and late.
Hearn had 10 points in the game's first 10 minutes, six on 3-pointers. The second of them, buried with just over 10 minutes left in the first half, put the Wildcats up 21-12.
And Northwestern made 26 of 31 free throws, many of them late with Illinois forced to foul.
The cold-shooting Illini (14-5, 1-4) closed within 48-39 on a free throw by Brandon Paul with 4:36 to play. Illinois would get no closer.
Paul had 21 points for Illinois, which shot 38 percent from the field, including going 3 of 20 from 3-point range.
Illinois opened the second half with two things that had been missing in the first, intensity and defensive pressure. Where first-half points sometimes came in easy waves for the Wildcats, early in the second Illinois made them work.
But with shooting cold -- they opened the second half hitting 4 of 11 from the field and were just 1 of 6 from 3-point range -- the Illini didn't make up any ground.
Marcotullio's 3-pointer with 12:53 left pushed Northwestern's lead to 45-30, the same 15-point margin they took to the locker room at halftime.
Illinois coach Jim Groce's frustration was apparent. He drew a technical foul for shouting at the officials with just under 11 minutes left in the game.
"That's enough -- you've got to calm down!" referee Jim Burr shouted at Groce along the sideline.
Swopshire missed both free throws, but the Wildcats didn't need the points.
Paul finally ended a scoreless run of just over 5 minutes for Illinois with a layup with 9:35 left to play, trimming the Wildcats' lead to 45-32.
From the opening moments, it was not the Illini's night, and there were any number of examples.
There was Nnanna Egwu's big block on Alex Olah's short jumper with 4:25 to play in the first half. With their team already down 30-17, the Illinois crowd seized on the moment, releasing a roar of collective relief. Before the noise had even faded, Olah grabbed the rebound and tucked it into the basket.
And there was the Illini's final first-half possession. Trailing 36-21, they had three chances to cut that lead and pocket something to build on in the final 20 minutes. But three chances yielded three misses, and the team trotted quietly off the court. The problem is, it hasn't been Illinois' night in a couple of weeks. Since they upset then-No. 8 Ohio State on Jan. 5, the Illini are 0-3. And there hasn't been a close game in the bunch, losing by 17 to Minnesota, a stunning 23 at Wisconsin and by 14 at home to Northwestern.
The Wildcats were coming off a blowout of their own at home against Iowa, 70-50, on Sunday. In that loss they couldn't hit their shots, finishing 15 of 51 (29.4 percent) from the field and 5 of 26 from 3-point range.
On Thursday, they made up for it, shooting 47.2 percent from the field. Hearn was 4 of 7 -- including 3 of 5 on 3-pointers -- and a cool 9 of 10 from the free throw line.