In addition to a career-high in points, Castle grabbed 12 rebounds and handed out 12 assists, coming up with three steals and a blocked shot for good measure.
The 2025 Rookie of the Year became at 21 the third-youngest player to record a 40-point triple-double, behind LeBron James and Luka Doncic. He joined Hall of Famer David Robinson as the only Spurs players with 40-point triple-doubles.
Castle was brutally efficient, connecting on 15 of his 19 shots to help the Spurs post their second win in three days over the Mavs, who have lost seven straight.
Devin Vassell added 17 points and Victor Wembanyama had 16 points and 11 rebounds for the Spurs, who led 81-67 at halftime.
Klay Thompson led the Mavericks with 19 points while sensational rookie Cooper Flagg finished with 14 - ending his streak of four games with 30 or more.
Flagg didn't play in the fourth quarter, with Mavs coach Jason Kidd saying after the game that the 19-year-old was battling a stiff back.
Castle reached 40 points with a put-back dunk midway through the fourth quarter that put San Antonio up 129-108.
"It felt good, but more importantly it felt good to do that knowing we were going to get the win as well," he said.
Rockets silence Thunder
The Houston Rockets, fueled by Alperen Sengun's triple-double and some timely baskets from superstar Kevin Durant, rallied to beat the short-handed NBA champion Thunder 112-106 in Oklahoma City.
Houston trailed by as many as 15 in the second quarter against a Thunder team missing reigning NBA Most Valuable Player Shai Gilgeous-Alexander.
But they chipped away to take the lead in the third period and held on down the stretch to snap a two-game losing streak.
"We started making shots," said Durant, a two-time NBA champion who launched his career with the Thunder franchise and heard boos from fans still miffed by his departure more than nine years ago.
"I think we got good looks that first half, we just weren't knocking them down. Second half, we knocked them down.
"We knew they would slow down a bit," Durant added. "They were playing with a lot of energy at home the first half. They were missing three guys, other guys get an opportunity (and) they took advantage in the first half. The second half, I think we came out with the same energy."
Durant, bottled up in a first half that saw him attempt just three shots, finished with 20 points on 6-of-10 shooting.
Turkish center Sengun scored 17 points with 12 rebounds and 11 assists and added three steals and three blocked shots. Jabari Smith Jr. had a double-double of 22 points and 10 rebounds.
Tari Eason led Houston's scoring with 26 points as all five Rockets starters finished with at least 16.
Cason Wallace led the Thunder with 23 points and Isaiah Joe added 21 off the bench for the reigning champions, who still boast the league's best record at 40-13.
Forward Chet Holmgren, back from a one-game injury absence, added 17 points and 14 rebounds, but the Thunder - missing Gilgeous-Alexander until after the All-Star break with an abdominal injury - couldn't maintain their early momentum.
In other early games, the Brooklyn Nets scored 80 points in the first half on the way to a 127-113 victory over the Washington Wizards, who had just eight players available amid injuries and the wait for newly traded players.
Michael Porter Jr scored 23 points, 20 of them in the first half, as the Nets halted a three-game skid.
A milestone night for Nikola Jokic helped the Denver Nuggets bounce back from a double-overtime loss to the New York Knicks with a 136-120 victory over the Bulls in Chicago.
Jokic scored 22 points, grabbed 14 rebounds and handed out 17 assists, his 182nd career triple-double moving him past Oscar Robertson for second on the all-time list headed by Russell Westbrook's 207.
Charlotte's Miles Bridges scored 26 points and rookie Kon Knueppel added 23 as the Hornets posted their ninth straight victory with a 126-119 win over the Hawks in Atlanta.
The Hornets' streak is the longest active streak in the league and the club's longest since they won nine in a row in 1999.
In Los Angeles, LeBron James scored 20 points with seven rebounds and 10 assists to lead the Lakers to a 105-99 victory over the Golden State Warriors.
Moses Moody led the Warriors with 25 points in a California clash that lost some of its luster with the injury absences of Warriors star Stephen Curry and the Lakers' Luka Doncic.