The junior defensive back -- nicknamed Honey Badger for his tenacious style, small stature (5-foor-9, 175 pounds) and blonde streak of hair -- rose from obscurity to become one of college football's biggest stars last season.
He was a sleeper Heisman candidate as the Tigers won the Southeastern Conference championship and reached the BCS title game. They are among the favorites to win the title this year, and remain so even without Mathieu.
At a news conference Friday, coach Les Miles would not specify the reason Mathieu was dismissed.
"We'll miss the guy," Miles said. "The football team's got to go on. We'll have to fill the void."
Athletic Director Joe Alleva said Mathieu, who was suspended for a game in 2011 after failing a drug test, violated an athletic department rule and had his scholarship revoked.
"Being an athlete is a privilege," Alleva said. "It's a privilege and you have to follow the rules to take advantage of that privilege. And unfortunately, he doesn't have that privilege here anymore.
"He really is a good kid. It's a shame. But I told him this morning that he has the rest of his life and his life is still ahead of him. He still has a tremendous opportunity to do good things and I encouraged him to do those good things, and I think he will."
Alleva said the violation did not involve law enforcement and Mathieu could stay at LSU and pay tuition. But Alleva said that was unrealistic.
"He's not going to stay in school," he said.
The Tigers are No. 1 in the coaches' preseason poll and expected to be right near the top of the AP college football poll when it is released Aug. 18.
They open the season at home Sept. 1 against North Texas. Their only big nonconference test comes the next week when Washington and star quarterback Keith Price visit Tiger Stadium. LSU opens SEC play on the road Sept. 22 at Auburn and renews its rivalry with Alabama on Nov. 3 in Death Valley.
The 20-year-old Mathieu won the Bednarik Award as national defensive player of the year last season and was a big-play machine at cornerback and special teams. The All-American scored four touchdowns -- two on punt returns and two on fumble returns -- intercepted two passes, caused six fumbles and recovered four.
LSU went 13-0 on its way to the BCS title game, and it seemed every time the Tigers needed to a game-changing play, Mathieu delivered.
The best examples came in the Tigers' final two victories. Against Arkansas to end the regular season, LSU trailed 14-7 when Mathieu brought back a punt 92 yards for a touchdown late in the first half.
The next week in the SEC title game against Georgia, the Bulldogs led 10-0 when Mathieu scored on a 62-yard punt return.
But as good as Mathieu was, LSU showed it could get by without him. He was suspended for the Auburn game and the Tigers won 45-10.
"He gave us a lot of examples that we can learn from and I think that he's a quality, quality guy who had a behavior issue and that's it," Miles said. "Certainly the overview of his time with us is positive."
Mathieu, a New Orleans native, has two years of eligibility left and could transfer, but he would have to sit out this season if he went to another school in major college football. If he moved down a level, to FCS, he could play right away.
"I can't imagine he would be here and not want to transfer and go play football," Miles said. "We will help him in every way we can."
LSU isn't quite as deep at cornerback as it was last season, when it had All-American Morris Claiborne and often used Mathieu as a nickel back. Tharold Simon is the other starter opposite Mathieu. Second on the depth chart are redshirt freshman Jalen Collins and freshman Jalen Mills.
As for returning punts, Miles mentioned wide receiver Odell Beckham as a possible replacement for Mathieu.
Even without the Honey Badger, LSU is loaded with talent.
"The good news is we have good players" to fill the void left by Mathieu, Miles said. Last season, LSU beat Alabama 9-6 in a regular-season matchup dubbed the Game of the Century. The two SEC rivals met again in the national championship but this time the Crimson Tide dominated, winning 21-0.