Dallas trailed in the third quarter before rallying.
The Mavericks took control early in the fourth quarter, scoring the first eight points and building a 110-100 lead on Barnes’ 3-pointer with 2:51 to play.
The Wizards shaved the lead to five, but Beal missed a 3-pointer and then Wall missed two 3s, any of which would cut the lead to two before Barnes made another 3 with 24.3 seconds left to put the game away.
The Mavericks went on a 21-7 run in the middle of the second quarter to take its biggest lead of the half at 54-43. But Wall took over the final 2:30 of the half, scoring 13 points in a 20-3 Wizards run that gave the Wizards a 67-61 lead at the break.
Wall scored the final 11 points of the half for Washington.
Wizards: Wall was named Eastern Conference player of the month for December after averaging 24.5 points, 10.7 assists and an NBA-best 2.7 steals per game as the Wizards went 10-5. . Washington has lost 12 of 13 to the Mavericks dating to 2009, their only win coming last season.
Mavericks: Andrew Bogut, who missed 11 games this year with a knee injury, limped to the bench with 3:52 remaining in the fourth quarter and did not return. . Forward Dorian Finney-Smith was a finalist for Western Conference rookie of the month for December but lost to New Orleans’ Buddy Hield. Finney-Smith started 16 of the team’s 17 games in December, averaging 6.6 points.
In an effort to promote the All-Star candidacy of Barnes, the Mavericks debuted the Harrison Barnes All-Star Burger at concession stands for all January home games. Barnes is averaging a team-leading 20 points for Dallas.
“I’m more concerned with his day to day progress, that he keeps learning and growing,” Mavericks coach Rick Carlisle said. “That’s really his personality so I think he’s going to continue to trend upward. He hasn’t plateaued in his work ethic or his thirst to get better.”
Wizards: Hosts Minnesota on Friday. Washington has won eight in a row at the Verizon Center.
Mavericks: Home against Phoenix on Thursday, the second game of a three-game homestand for a Dallas team that has played six of previous seven on the road.