The son of racer Martin Truex Sr., Martin Jr. began his racing career driving go-karts at the New Egypt Speedway located in Central New Jersey, when it was still a paved track (the track was later switched to dirt). Truex Jr. would make his move to the Modified division at Wall Stadium in 1998, as soon as he was old enough to race a car at the age of 18 (under New Jersey regulations).

Truex was born in Trenton, New Jersey, and grew up in the Mayetta section of Stafford Township, New Jersey. He graduated from Southern Regional High School in 1998. His father, Martin Truex Sr., was a former race winner in the Busch North Series. His younger brother, Ryan, is a former champion in the K&N Pro Series East, and was a contender for 2014 Rookie of the Year in Sprint Cup.

In 2000, Truex moved south and rented a home from Dale Earnhardt Jr. in Mooresville, North Carolina, eventually purchasing his own home there. Following in his father’s footsteps, he began racing in the Busch North Series. He ran three full seasons (2000 to 2002) and made limited starts in 2003. Truex claimed 13 poles and 5 wins driving his family-owned No. 56 SeaWatch Chevy.

Truex made his first Busch Series start in 2001 at Dover International Speedway in his father’s No. 56 Chevy. He started 19th but finished 38th after an early wreck. In 2002, Truex drove one race for Phoenix Racing at New Hampshire International Speedway, starting 13th and finishing 29th. He ran three races the rest of that season for his father, his best finish seventeenth at Dover.

In 2003, he began the season with his father’s team, before he was hired by Dale Earnhardt Jr. to drive his No. 81 Chance 2 Motorsports Chevy. He made his debut with Chance 2 at Richmond International Raceway, where he qualified sixth and led 11 laps before transmission failure forced him to a 31st-place finish. He split time between Chance 2 and his father’s team for the balance of the season, except at Dover, where he drove for Stanton Barrett. He had a sixth-place run at Bristol Motor Speedway, and ended the season with two consecutive second-place finishes. He ran a total of ten races that season.

Truex raced full-time for Chance 2 in 2004. At Bristol Motor Speedway, he would earn his first career victory, and he would later add three more victories over the next seven races. This would include a victory at Talladega Superspeedway which broke his car owner’s streak of winning restrictor plate races in the Busch Series, and a victory at the final NASCAR event held at Nazareth Speedway. He took the lead in the championship after Nazareth, but lost it to rookie Kyle Busch a few races later. However, a series of Top 5’s and Top 10’s in the second half of the season would allow Truex to pull away from Busch, clinching the Busch Series championship with a race to spare.

Truex stayed in the Busch Series to defend his championship in 2005, winning the title for the second season in a row. He won the first Busch Series points race held outside the United States, in Mexico, as well as defending his wins at Talladega and Dover International Speedway. He took his first win at Daytona International Speedway on July 1, 2005.

In 2006, Truex moved to the No. 1 DEI Chevy full-time in the Nextel Cup Series. He had two Top 5 finishes and finished 19th in points. Truex got his first win of the 2007 season in the NASCAR Nextel All-Star Open, securing a spot in the 2007 Nextel All-Star Challenge, where he finished tenth.

Truex and his girlfriend Sherry Pollex have been together since 2005. In 2007, they started the Martin Truex Jr. Foundation to support children with pediatric cancer. In 2014, Pollex was diagnosed with Stage III ovarian cancer.

In January 2007, Truex was charged with disorderly conduct after being spotted urinating on his car in a public parking garage by a police officer. Truex, who had been drinking at a bar with members of his team, was fined $100 and issued a public apology.

This victory led to a jump in overall points advancing him to 13th, followed by a third-place finish at Pocono Raceway and a second-place finish at Michigan. With a 15th-place finish in the Chevy Rock and Roll 400, Truex clinched a spot in his first Chase for the Sprint Cup and finished 11th in points at season’s end. He did not go to victory lane in 2008, but he did have 11 Top 10’s and finished 15th in the points standings.

2009 would be Truex Jr.’s lone season with Earnhardt Ganassi Racing, as he departed following the season and was replaced by Jamie McMurray.

At Martinsville the following year in 2011, a stuck throttle caused Truex to make contact with Kasey Kahne, resulting in a large wreck; Truex hit the wall hard head on and his car flew on fire for a few seconds as Kahne hit the wall in his car’s rear. Truex climbed out instantly and went to check on Kahne who received standing ovation as he climbed out uninjured. Truex then left with officials to the care center. He said the wreck was his hardest of his career and both Truex and Kahne were released with normal symptoms. The next week officials told him his wreck was the hardest crash at Martinsville. Truex won another pole at Dover International Raceway. Truex ran well in most of the races but often struggled to finish the races off. He was docked 25 points because of a windshield violation in the fall Talladega race. He ended the season 18th in points, with three Top 5’s and 12 top 10’s.

Truex started 2012 well, winning a $200,000 bonus and finishing 7th in the Daytona 500. He finally hit his stride in Texas, winning the pole and leading 69 laps. The following week at Kansas he started sixth and flat-out dominated the race, leading 173 of 267 laps but falling short to Denny Hamlin. At Atlanta, he led 40 of the final 46 laps, but ultimately fell short to Hamlin again when he was forced to pit under a late race caution for fuel, ending up fourth. Truex finished 21st, however he had a spot clinched in the Chase, and ultimately made it in. He ended up 11th in the points, with zero wins, seven Top 5’s, and 19 Top 10’s.

On October 14, 2013, it was announced that Michael Waltrip Racing’s No. 56 would become a research and development team in 2014, and later was spun off. Truex was told he could offer his services as a driver to other teams, and on October 17 it was confirmed that for the 2014 NASCAR Sprint Cup Series season Truex would move to Furniture Row Racing and drive the No. 78 Chevrolet being vacated by Kurt Busch. When the deal was formally announced on November 1, 2013, before the Texas race, it was announced that Furniture Row had also signed on all of the members of Truex’s MWR pit crew.

Truex started his first season with JGR with a 35th-place finish at the 2019 Daytona 500, but he made up for that loss with five straight top-10s and two top-20s before scoring four wins at Richmond (his first Cup win on a short track), Dover, Charlotte, and Sonoma. He began the 2019 playoffs by winning the first two races at Las Vegas and Richmond (his first back-to-back career victories and season “sweep” at Richmond) and advancing to the Round of 12 after finishing seventh at the Charlotte Roval. During the Round of 8, Truex won at Martinsville to secure his position in the Championship 4. At the season finale at Homestead-Miami Speedway, Truex dominated the first half of the race, winning the first stage and leading into green flag pit stops at the midpoint of stage 2. However, the race and championship hopes unraveled due to a miscue during the pit stop, in which the left and right front tires were switched around, and installed to the incorrect sides. The pit road error forced an unscheduled stop and rendered Truex a lap down for much of the second stage. He recovered and began to rally back through the field, but could not overcome the loss of track position, ultimately finishing second in the race to teammate Kyle Busch and in the final championship standings for the second season in a row. Truex led the series in victories with seven. Truex was also the first driver to lead the series in wins after switching to a new team since Matt Kenseth in 2013, who also did it with Joe Gibbs Racing.

Truex’s 2014 season started with an outside pole qualifying run for the Daytona 500, Furniture Row Racing’s first front row start in the 500. However, Truex’s engine failed on lap 31.

2016 became his first multi-win season of his career. Truex would advance to the next round of the Chase at Chicagoland. Truex led 32 of the final 50 laps and appeared set to finish second behind Chase Elliott. After a caution with 10 laps left erased Elliott’s 3-second lead, Truex and Elliott pitted. Truex used advantage of his fresh tires and got by rookie Ryan Blaney on the final restart, winning the race, his third win of the season. At New Hampshire, Truex had a great car all day and would lead the most laps but towards the end his tires would wear, and a few late race cautions came out so he would lose the lead, eventually, Kevin Harvick would win and Truex would finish 7th.

At Kansas in May 2017, Truex started 4th. The race was primarily a duel between him and Ryan Blaney. Truex held off Blaney, Kevin Harvick, and a hard-charging Brad Keselowski on three restarts to win the race. At Charlotte, he led the most laps once again, becoming only the second man to lead the most laps in three straight Coca-Cola 600 races, yet only winning one. He tied Darrell Waltrip’s record. At Michigan, he won 2 more stage wins, becoming the first and fastest person to ever win 10 stage wins, where nobody else has won more than four, or any other team’s combined stage wins. In July, Truex captured his third win of the season, dominating the Quaker State 400 at Kentucky Speedway. He again won all three stages, leading 152 of 267 laps. With two laps to go, Truex had a 14-second lead before a late caution set up an overtime finish. Despite each of the other seven cars behind him pitting during the caution, Truex held off Kyle Busch and Kyle Larson on older tires before a wreck behind the lead pack brought out another yellow flag, this one ending the race.

On September 4, 2018, it was announced that Furniture Row Racing will be folding the No. 78 team following the conclusion of the 2018 season. On November 7, 2018, with Furniture Row Racing closing at the end of 2018, it was announced Truex and crew chief Cole Pearn signed a deal with Joe Gibbs Racing to drive the No. 19 starting in the 2019 season, replacing Daniel Suárez and crew chief Dave Rogers.