GREENSBORO, N.C. (AP) — Marcus Thornton wasn’t around for LSU’s last NCAA tournament appearance, stuck instead at a junior college while those other homegrown Tigers reached the Final Four.

He finally got his chance on the sport’s grandest stage, and played like he plans on hanging around for a while — especially when upset-minded Butler threatened to end things.

Thornton scored 30 points while coming through with one critical basket after another for hot-shooting LSU, which held off a late rally to beat the Bulldogs 75-71 on Thursday in the first round of the NCAA tournament.

Tasmin Mitchell had 14 points and Chris Johnson added 12 points and four blocked shots for the Tigers (27-7), seeded eighth in the South Regional. They built an early 13-point lead and shot 49 percent against one of the nation’s toughest defenses in advancing to Saturday’s second round against the North Carolina-Radford winner.

‘‘Did we think we were going to blow them out? We haven’t blown anybody out all year long,’’ coach Trent Johnson said. ‘‘We have a couple of guys that can make some plays, and when those guys don’t make plays, we have some other guys that are not afraid to step up and shoot the ball.’’

Matt Howard scored 22 points before fouling out with 35.7 seconds left, and Shelvin Mack added 18 points for No. 9 seed Butler.

The Bulldogs (26-6) made things interesting by twice pulling within three in the final minute, the last coming when Willie Veasley tipped in Gordon Hayward’s missed 3 with 5.4 seconds left, making it 74-71. Johnson hit one of two free throws with 4.5 seconds left to seal it.

Hayward, Butler’s top 3-point shooter, finished with 12 points for the Bulldogs. The Horizon League regular-season champions were beaten in the first round for the first time since 2000.

‘‘Of anything you take from this game, you no longer have the chance,’’ Butler coach Brad Stevens said. ‘‘The chance is now gone, and, you know, nothing is guaranteed. For us, we have to be terrific in the regular season if we don’t win our conference tournament, and we recognize that. So you have to maximize every day.’’

These Bulldogs nearly did.

Considered one of the nation’s most dangerous mid-majors, they had knocked off Maryland, Wake Forest, Louisville and Mississippi State in past tournaments. And playing with an inexperienced lineup that included three freshman starters in what was considered a rebuilding year, they shook off a rough first half and moved into position to add LSU to that list when Zach Hahn hit a free throw to tie it at 58 with 5:15 remaining.

But Terry Martin stuck back Garrett Temple’s missed 3-pointer 15 seconds later, and Thornton knocked down a 3 from the right wing at the 4:08 mark to give LSU some breathing room, and the defense kept the Bulldogs at bay until the wild final minute.

‘‘Team defense, that’s how you win games,’’ Thornton said. ‘‘We locked up at the end of the stretch.’’

Maybe so, but those big shots he hit also helped. The senior guard establish himself as the biggest reason why the Tigers are in the tournament for the first time since they advanced to the national semifinals three years ago.

One of three Baton Rouge natives in the starting lineup, Thornton could have been a key member of that Final Four team had academic problems not led him to a junior college in Texas while Mitchell and Temple logged meaningful minutes.

Certainly, Thornton is making up for lost time.

The Southeastern Conference’s player of the year was 10-of-15 from the field and hit three of his four 3-pointers in his first 30-point game since last month against Georgia. Five of his baskets came when it was a one-possession game in the second half, including an impressive turnaround jumper with 14 1/2 minutes left that put the Tigers back up 43-42.

‘‘We have a play call, you know, we’ve been running it all year and nobody can stop it,’’ Thornton said. ‘‘So, I just took whatever they gave me.’’

At times, Thornton had his way with a tough Butler defense that entered allowing opponents to shoot just 38 percent.

‘‘I felt if we could hold them under 70, we’d win,’’ Stevens said. ‘‘We just didn’t do that.’’

The Tigers dominated early, seemingly out to prove that they were underseeded. Their long, athletic defense — led by Johnson, a 6-foot-11 center — played havoc with Butler’s deliberate, space-conscious offense by holding the Bulldogs to one field goal during the first 11 minutes.

At the 8-minute mark, LSU had blocked more shots (three) than Butler had made (two).

‘‘We never got overwhelmed with the lead we had in the first half, because the game’s never over,’’ Mitchell said. ‘‘We were going into the game hoping we could get the big lead on them, but this is the NCAA tournament.’’

Johnson eventually wound up in foul trouble, and the Bulldogs got themselves back into the game. The shot-altering big man picked up his third foul during Butler’s surge of nine straight points, and Veasley’s alley-oop dunk with 15 minutes left made it 42-41.

‘‘We told Chris to come out aggressive, don’t worry about the fouls,’’ Mitchell said. ‘‘Just be careful, but always play with your intensity, and that’s what he did. He came back out with three fouls, played like he didn’t have none.’’