Law was drafted 23rd overall in the first round of the 1995 NFL Draft by the New England Patriots where he spent the majority of his career. In 1998 Law earned his first trip to the Pro Bowl. That year he led the league with nine interceptions, the first time any Patriot had led the league in interceptions.
Law earned his first Super Bowl ring with the Patriots in 2001. In Super Bowl XXXVI, Law intercepted a Kurt Warner pass and returned it 47 yards for a touchdown, the first points of the game for the Patriots, who eventually won the game 20-17.
Law was voted to the Pro Bowl for the second consecutive year and for the fourth time in his career after the 2003 season. In 2003, Law was part of a record-breaking Patriots defense that led the NFL in four key categories: opponents’ points per game (14.9), opponents’ passer rating (56.2), interceptions (29) and passing touchdowns surrendered (11). Law's physical play against some of the game's best receivers prompted the NFL to more strictly enforce the five-yard illegal contact rule on defensive backs after the 2003 season. In the AFC Championship Game against the Colts, Law intercepted 3 passes from Colts quarterback Peyton Manning, assisting his team to a 24-14 win and their second Super Bowl appearance in 3 years, where they defeated the Carolina Panthers 32-29.
Law earned his third Super Bowl ring with the Patriots in 2004, but missed the final 9 games of the season and all three of the Patriots' playoff games due to a foot injury.
On February 25, 2005, Law was released by the Patriots due to his $12,551,000 cap salary.
Law signed with the Jets for the 2005 season. He amassed a career-high 10 interceptions and was the only Jet voted into the Pro Bowl (Jonathan Vilma was named to the Pro Bowl as an injury replacement to Miami's Zach Thomas, not by means of popular vote by the fans). Law was released by the New York Jets on February 22, 2006 as the Jets were a projected $26 million over the salary cap for 2006. Law was due to make $7.6 million for 2006.
On July 25, 2006, Law passed his physical with the Chiefs and signed a five-year deal worth $30 million. Law reunited with coach Herman Edwards, whom Law had played under in the 2005 season hoping to strengthen the Chiefs' defense.
Law signed with the Denver Broncos on November 7, 2009. This added to a defensive backfield that had 5 members over 30 years of age, with 20 Pro Bowl selections combined. His final game with the Broncos came January 3, 2010. He finished the season with 10 tackles, and 1 interception run back for 37 yards. He was released by the Broncos on February 24, 2010.