Lawrie, the second-round leader, fired a seven-under 65 on Sunday. The Scotsman was tied for the lead after eight holes, but played the last 10 in six-under par to earn a four-shot win.
"I don't think I can play much better than that," said Lawrie, who finished at 15-under 201. "I've been playing well for a long, long time, but it's just nice to come out one ahead and shoot seven-under."
Day (65) and Peter Hanson (67) shared second at 11-under 205.
John Daly carded a five-under 67 and took fourth alone at minus-nine.
Garcia, who was tied for the lead with Lawrie after a birdie at the 10th, notched one birdie and two bogeys the rest of the way. He fell into a tie for fifth with Jean-Baptiste Gonnet (65), Soren Hansen (66) and Ricardo Gonzalez (70) at eight-under par.
The victory was Lawrie's seventh on the European Tour. He captured the 1999 British Open, but earlier in that season, he won this championship.
"Now wouldn't that be nice to get that again?" he asked.
Lawrie returned to the winner's circle for the first time in nine years when he won last year's Open de Andalucia.
Lawrie is expected to move into the top 45 in the world rankings based on this win. He hasn't been in that position since 2003.
The tournament was shortened to 54 holes when most of Friday's action was called off due to high winds.
Lawrie grabbed the second-round lead with a 67 in round two, then birdied his first hole Sunday morning. He parred his next seven and Garcia took that opportunity to vault up the leaderboard.
Garcia birdied one, five, seven and eight to get within one. His birdie at the par-five 10th tied him for the lead with Lawrie at nine-under par, but Lawrie chipped in for eagle at the par-five ninth to move two clear.
Garcia got one back with birdie at the 11th, but that was the last threat the Spaniard mounted. He three-putted the 13th for a bogey, dropped a shot at 15 and fell several behind Lawrie.
The leader played the best golf of the round after the eagle at nine. Lawrie ran home a six-footer for birdie at the 11th and was three in front of Garcia and Day, who birdied four in a row from the 10th.
Day, the No. 10 player in the world, trimmed the gap to two, and Lawrie began to spray the ball off the tee. He had a decent look at birdie at the 13th, but extended his cushion back to three with a birdie at 14.
Lawrie missed a good opportunity for birdie at 15, but cashed in at 16. He drove into the left rough at the drivable par four, then chipped to three feet to set up the easy birdie.
Four ahead with two to play, Lawrie birdied the par-three 17th and the final margin of victory became four only when Day birdied the par-five last.
With a four-shot edge to play with, Lawrie laid up at the closing hole and made a par to secure the trophy.
"When you've got a chance to win a tournament you don't sleep as well the night before and things go racing through your mind," said Lawrie. "You've got to get back to basics and I did that. I hit some nice shots coming in."
Kaymer was joined in ninth place by Victor Dubuisson (69) and Nicolas Colsaerts (72). The trio finished at seven-under 209.
NOTES: Lawrie has three top 10s in his first three starts of the season...European Ryder Cup captain Jose Maria Olazabal celebrated his 46th birthday Sunday with a three-under 69. He tied for 12th, his second top-15 finish of the season...Last year's winner Thomas Bjorn posted a four-under 68 on Sunday and tied for 25th at four-under...Next week, the tour hosts the Dubai Desert Classic, where Alvaro Quiros won the 2011 event.