LSU (55-16) would win its sixth national title with a victory over the Longhorns (49-15-1) on Tuesday night.
LeMahieu walked leading off the 11th against Brandon Workman (3-4), stole second with two outs and took third when catcher Cameron Rupp's throw went into center field. After Micah Gibbs walked, Mahtook singled into center field for the go-ahead run.
Matty Ott (4-2) struck out Brandon Loyn and Tim Maitland and got Connor Rowe to ground out to finish off Texas, which had walkoff wins in two of its first three CWS games.
The Tigers, who won their 14th straight, came back from deficits of 3-1 and 6-4. LeMahieu hit the tying, two-out double in the top of the ninth off Austin Dicharry. LeMahieu also homered in the seventh.
Texas, trying to become the first No. 1 national seed since Miami in 1999 to win the championship, continued its show of power. Russell Moldenhauer hit two of the Longhorns' five homers.
The Longhorns, who came to Omaha with 39 homers in 61 games, have hit 11 in four games here.
All three of Moldenhauer's homers this season have come in the CWS. He was one of three Longhorns to go deep in the fourth inning, and he broke a 3-3 tie in the sixth with his opposite-field shot to left.
Travis Tucker and Kevin Keyes also homered in the fourth, and Rowe did it in the seventh.
All five Texas homers were off LSU starter Louis Coleman, who was pulled after Rowe's shot.
The Longhorns didn't get another hit off Chad Jones, Paul Bertuccini and Ott, who worked the last three innings.
In the ninth, Sean Ochinko singled off Austin Wood and Derek Helenihi was walked by Taylor Jungmann. Dicharry came on and struck out Tyler Hanover before LeMahieu doubled into the left-field corner to tie it.
Texas, which plays its home games at the cavernous UFCU Disch-Falk Field in Austin, is better known for playing small ball. The Longhorns are only the fifth team to record more than 100 sacrifice bunts in a season.
But with the wind blowing out at Rosenblatt Stadium, the Longhorns have showed themselves capable of going deep.
In fact, over a span of 6 1-3 innings from their previous game against Arizona State to Monday's matchup with LSU, seven of the Longhorns' 11 hits were home runs.
Texas is the first team since LSU in 1998 to homer three times in an inning at the CWS.
Tucker tied it 1-1 with his third of the year, driving a 1-1 pitch into the first row of the left-field seats. Moldenhauer lofted Coleman's 1-0 pitch off a flag pole well over the 22-foot wall in center field. Keyes hit a first-pitch homer into a section of Texas fans in left center for a 3-1 lead.
Chance Ruffin, who struck out 10, left with two out in the top of the sixth with runners at first and third so the lefty Wood could be matched against left handed-batting Jared Mitchell. The strategy backfired as Mitchell tripled into the left-center gap to tie it.
But Moldenhauer homered in the bottom half and Keyes scored on a wild pitch to put the Longhorns up 5-3.
Ryan Schimpf opened the scoring with his third homer of the CWS, and 22nd of the season, in the first.