It means Porter earned the vacant WBC welterweight after previously holding the IBF title until he was beaten by British fighter Kell Brook in 2014.
American Porter, 30, impressed with his brawling style and won with a unanimous decision.
The judges scored the fight 116-112, 115-113 and 115-113 in Porter's favour as the Ohio-born man won his 29th professional boxing fight.
Garcia started strongly and had the better of the early rounds before Porter started to take control of the fight.
He attacked the Garcia body, freezing and feinting to confuse his opponent and landed 180 punches to Garcia's 168.
Porter said: "I tell people all the time, 'I don't make predictions'. I made a prediction and a hard one to live up to.
"I said I wasn't leaving New York without this belt, and I'm not leaving New York without this belt.
"I knew he was going to be accurate. The game plan for me was to be accurate from the outside and show we could beat him without roughing him up on the ropes. He tried to out hustle me, mostly at the end of rounds. He did a tremendous job.
"There was some things missing tonight and those aren't going to be there in the next one. It wasn't necessarily about making it wild. My dad [trainer Kenny Porter] wanted me to stay consistent with the body work and stay consistent with the pressure."
And after the fight, IBF champion Errol Spence Jr joined the WBC holder in the ring and the pair embraced amid cheers from the fans.
Spence beat Brook in May 2017 to win the belt but Brook may face Amir Khan in his next fight, after the Bolton boxer beat Samuel Vargas and admitted Brook would be his second-choice fight, behind Manny Pacquiao.
Spence has a 24-0-0 record in professional fights and looks on course to take on Porter in what would be a mouth-watering clash.
They said they would like to secure a unification bout and Porter claimed it would be "the easiest fight in boxing to make".
The IBF champion said to Porter: "The same way that you called Danny out, I'm going to call you out.
"I think I'm the best welterweight in the division. I'm the truth, and I guarantee you I come home as unified champion. I definitely want that fight against Porter.
"It was a good fight. Shawn looked like himself. I didn't see anything different in his style so we should combine to make an incredible fight.
"I'm ready for whoever's next: Mikey Garcia, Shawn Porter or Keith Thurman. I want any and all of them."
WBA champion Thurman was present at the Barclays Centre for the fight but was booed by the 15,000-strong crowd.
He held the WBA and WBC world titles after beating Porter and Garcia, the latter in March 2017, but has not fought since due to injury and subsequently been stripped of his belts.
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