Williams was born in Highland Hills, Dallas. He attended W. T. White High School in Dallas, and played for the White Longhorns high school football team. As a senior, he had 59 receptions for 972 yards and eight touchdowns. Considered a two-star recruit by Rivals.com, he accepted a scholarship to Baylor over an offer from Colorado State.
Williams attended Baylor University, where he played for the Baylor Bears football team from 2008 to 2012. During his college career, he had 202 receptions for 3,334 yards with 27 touchdowns. Williams was also a Biletnikoff Award finalist for 2011. In 2012, he led the nation with 1,832 receiving yards.
In the 2013 NFL Draft, the Dallas Cowboys traded down in the first round with the San Francisco 49ers, in exchange for a third round pick that the team used to select Williams with the 74th overall pick.
Williams struggled in training camp adjusting to the team's offense, but by the end of the preseason he had supplanted Dwayne Harris as the third wide receiver. He again struggled at the start of the regular season with his route running and dropped passes, until the fourth game against the San Diego Chargers, where he started in place of an injured Miles Austin and registered seven receptions for 71 yards, but also had a critical fumble trying to extend for a touchdown in the last minutes of the game. He started the next week playing against the Denver Broncos and had four receptions for 151 yards with one touchdown. By Week 8 against the Detroit Lions, Williams set a Cowboys' franchise record for rookies, with a touchdown in his 4th consecutive game. He returned to his third receiver position in the 11th game of the season against the New York Giants and also started returning kickoffs in place of an injured Harris.
He finished his rookie season with 44 receptions for 736 yards (16.7 average) and 5 receiving touchdowns. At the time, his totals ranked fourth for most receptions, most receiving yards and most receiving touchdowns by a rookie in franchise history.
Williams started the year looking to improve on a productive rookie campaign, immediately establishing himself as a legitimate No. 2 wideout and deep threat opposite Dez Bryant by catching 6 touchdowns in the first 7 weeks. After a strong start to his season that included a critical first down reception in the Seattle Seahawks win, he slowed down catching only 2 touchdown passes in the remaining 10 weeks, finishing with 37 receptions for 621 yards and 8 touchdowns.
During the first playoff game of the Cowboys season, and his career, Williams once again solidified his status as a playmaker by registering 3 receptions for 92 yards, while scoring 2 critical touchdowns against the Detroit Lions and their No. 2 ranked defense. This included a 76-yard catch-and-run in which he caught a 14-yard pass on a slant route and ran through 4 defenders en route to a 76-yard touchdown as well as the game-winning 8 yard score, Dallas won the game 24-20. In the next playoff game against the Green Bay Packers, he drew a pass interference in the first quarter that led to a touchdown and also scored on a 38-yard touchdown reception, but he was a non-factor in the second half of the game, in which the Cowboys lost 21–26.
With leading wide receiver Dez Bryant missing most of the season, Williams failed to show that he was ready to be a number one receiver, which was complicated by the fact that he played with 4 different starting quarterbacks with varying degrees of knowledge of the team's offense.
He had a career-game statistically in the season finale against the Washington Redskins, making 8 receptions for 173 yards in a 34-23 loss with Kellen Moore as the starting quarterback. He only had two games where he registered over 80 receiving yards and finished with a total of 52 receptions for 840 yards and 3 touchdowns.
In the Cowboys' 2016 home opener against the Giants, Williams caught the ball near the end of the game but did not run out of bounds allowing the Giants to win the game 20-19. Williams caught his first touchdown of the season against the 49ers in Week 4. In the regular season finale on January 1, 2017 against Philadelphia Eagles, Williams caught a touchdown on what turned out to be the final play of Tony Romo's career. Williams finished the season with 44 receptions for 594 yards and 4 touchdowns.
On March 10, 2017, Williams signed a four-year, $17 million contract extension with the Cowboys.