Law was drafted by the Texas Rangers in the 28th round of the 2009 Major League Baseball draft out of Seton-La Salle Catholic High School in Pittsburgh, Pennsylvania. A two-time Pittsburgh All-Area baseball selection, he did not sign and attended Miami Dade College. In his sophomore year at Miami Dade, Law posted an 8–5 record with a 2.35 ERA while striking out 121 batters in 92 innings. He was then drafted by the San Francisco Giants in the ninth round of the 2011 Major League Baseball Draft. He made his professional debut with the Arizona League Giants in 2011. In 15 games he had a 2.50 earned run average with 19 strikeouts over 18 innings pitched. In 2012, he pitched for the Augusta GreenJackets, posting a 2.91 ERA with 67 strikeouts over innings. Law spent the 2013 season pitching for the GreenJackets, AZL Giants and San Jose Giants. He finished the year with 2.31 ERA, 14 saves and 102 strikeouts over innings. At San Jose, Law struck out 45 hitters while only walking one hitter. Prior to the 2014 season, he was invited to the Giants Spring Training, where he has competed for a spot in the Giants Opening Day bullpen. In 2014, he pitched for the Double-A Richmond Flying Squirrels, recording 13 saves in 14 opportunities and striking out 29 in 28 innings pitched. Law underwent Tommy John surgery in June 2014 and did not return until June 2015. Law was added to the Giants' 40-man roster in November 2014 to protect him from the Rule 5 draft. In 2015, Law recorded 32 strikeouts in 25 innings for the Flying Squirrels. Law started the 2016 season with the Triple-A Sacramento River Cats. Law was called up to the San Francisco Giants on April 15, 2016, to replace the injured Sergio Romo. Law made his major league debut in a gamelater that day against the Los Angeles Dodgers. He entered the game in the bottom of the seventh inning and allowed one hit while striking out three batters. On May 7, Law earned his first major league victory by holding the Colorado Rockies scoreless in one inning of relief. On June 13, Law pitched 1⅓ scoreless innings against the Milwaukee Brewers to record his first major league save. In 61 games, he posted an ERA of 2.13 in 55 innings. The following season, he struggled with inconsistency, posting an ERA of 5.06 in innings. In 2018, he only appeared in 7 games for the Giants, spending most of his time in the AAA level.
His father, Joe Law, played minor league baseball in the Oakland Athletics organization. He was called up to the majors for four days but never appeared in a game.
