Garcia moved up two weight classes on Saturday night to take on Spence and took a pounding, though he never went down. Garcia, obviously worn out, spent the later rounds blocking punches instead of throwing them.
Spence improved to 25-0 after all three judges awarded him every round. One card was 120-107, and the other two were 120-108.
It was the first time Spence had to go the distance in his last 12 fights.
Garcia, who moved up from 135 to 147 pounds in his attempt to become a five-division champion, lost for the first time in 40 pro fights.
The hard-punching Spence landed 345 of the 1,082 punches (32 per cent) he threw both of those career highs. That included 237 of 464 (51 per cent) of his power punches in his second title defence at home in North Texas in nine months.
At one stage during the ninth round, Spence walked Garcia in a full circle around the ring while continually delivering blows.
Garcia was fighting for the first time since unifying the IBF and WBC world lightweight titles with a unanimous decision last July over Robert Easter in Los Angeles. He was trying to become a five-division champion.
"Throughout training camp, a lot of commentators thought he was too smart and I couldn't box as well as him. I showed I can box and I can move my head if I want to," Spence said.
"The game is to be smart. It's the sweet science. I had the size and reach advantage, so why not use it to take away the jab? It's a weapon for me and it takes away one of his weapons."
"The motivation fighting in front of my hometown crowd made me feel great," Spence said. "These people have supported me since day one and I wanted to put on a good performance for all of them."
WBA world welterweight champion Manny Pacquiao, a five-division champion, was at the AT&T Stadium and could be a future opponent for Spence. Pacquiao won two title bouts at the Cowboys stadium in 2010, when he beat Joshua Clottey for the WBO welterweight crown and later Antonio Margarito for vacant WBO super welterweight title.
The 40-year-old Pacquiao wants to return to the ring in July.
