Garcia, who beat Amir Khan in a fourth-round knockout three months ago, outclassed Mexican legend Morales as he knocked him out in the fourth round, controlling the action against an opponent who was both older and smaller.
Morales, a title holder at four different weights in a glittering career, battled away gamely but was undone by a hard shot in round four which sent him to the canvas and out of the fight.
Garcia now has 25 wins in 25 fights, 16 of them by knockout.
After the bout Morales confirmed that he is hanging up his gloves, and will box only once more in a farewell bout in his homeland.
On the same card Paulie Malignaggi defended his WBA welterweight title against Pablo Cesar Cano, getting the benefit of a controversial split decision in front of his home crowd.
Cano appeared to have done enough after opening up a cut above Malignaggi's left eye early on and then dropping the champion in the 11th round with a fearsome right, but judges scored it 114-113, 114-113 and 109-118 in Malignaggi's favour.
Also in Brooklyn Peter Quillin won the WBO middleweight title and kept his perfect record intact as he conquered previously unbeaten Hassan N'Dam N'Jikam.
The fight went the distance, but Quillin was an overwhelming winner as he dropped his French-Cameroonian opponent no fewer than six times - an impressive display against a fighter who had never before been sent to the canvas in his professional career. Quillin won 115-107, 115-107, 115-107.
And finally Devon Alexander won the IBF world welterweight crown with a unanimous points victory over reigning champion Randall Bailey, with both fighters disappointing in a dull bout but Alexander outscoring Bailey 115-111, 116-110 and 117-109.