Despite crashing out early at Daytona, Dillon got his first career pole at Texas Motor Speedway in June and finished third, leading 20 laps. On July 11, Dillon scored his first career NASCAR victory in the Lucas Oil 200 at Iowa Speedway, winning a Truck Series race in a truck wearing the No. 3 for the first time since Bryan Reffner won for Team Menard in 2000 at Texas Motor Speedway. He ended the season with two wins and seven poles, earning him Rookie of the Year honors.

He made his first Camping World Truck Series start at Iowa Speedway, driving the No. 3 truck. This was the first time that the No. 3 had appeared in any of the three major series since Dale Earnhardt Jr. used it in the Busch Series back in 2002.

Dillon was supposed to start the Mountain Dew 250 at Talladega Superspeedway during the 2009 NASCAR Camping World Truck Series season, but the truck failed inspection, thus his time was removed and he was not allowed to race. Dillon would drive the No. 3 truck full-time in 2010 for Rookie of the Year, with sponsorship from Bass Pro Shops.

In 2011, Dillon scored his first win of the year at Nashville Superspeedway. Dillon scored his second win of 2011 at Chicagoland Speedway, beating Kevin Harvick and Kyle Busch. After rain shortened the season finale at the Homestead-Miami Speedway, Dillon was crowned the 2011 NASCAR Camping World Truck Series Champion. He also won the 2011 CWTS Most Popular Driver Award. In 2012 he moved up to the Nationwide Series full-time, driving the No. 3 Chevrolet for RCR.

Dillon made his first career start in the NASCAR Sprint Cup Series on October 9, 2011, in the Hollywood Casino 400 at Kansas Speedway. He finished 26th in his No. 98 Camping World Curb/Agajanian Racing Chevrolet Impala.

On November 4, 2011, Richard Childress Racing announced that Dillon would be moving up to the Nationwide Series in 2012, driving the No. 3 that Dale Earnhardt made famous, with sponsorship from AdvoCare, American Ethanol, and Bass Pro Shops.

Dillon competed in the Sprint Cup Series on a limited basis in 2012, starting at Michigan International Speedway in June, driving the No. 33 Chevrolet for Richard Childress Racing, starting 21st and finishing 24th.

On June 29, 2012, Dillon recorded his first Nationwide Series win at Kentucky Speedway; he dominated the race by leading all but 8 laps and won by over 9.8 seconds over second place. However, NASCAR officials announced the car had failed post-race inspection because the rear ride heights were too low.

Dillon also participated in one race in the NASCAR Canadian Tire Series at Circuit Gilles-Villeuneuve in Montreal in August 2012. Dillon finished 7th while driving the No. 4 car for Fitzpatrick Motorsports.

Dillon announced in August 2012 that he would continue to drive full-time in the Nationwide Series for 2013, with Advocare as the primary sponsor for the full season, along with running seven Sprint Cup Series races that year, including the 2013 Daytona 500, where he qualified 8th, but crashed late in the race, relegating him to a 31st-place finish. In January 2013 it was announced that some of Dillon’s Sprint Cup races that season would be in Phoenix Racing’s No. 51 Chevrolet.

On November 16, 2013, Dillon won the NASCAR Nationwide Series Championship with a 12th-place finish in the season finale at Homestead-Miami Speedway. Dillon’s series championship was unusual because he did not win any races during the season; this marked the first time in any of NASCAR’s three national series that a season champion went winless.

On December 11, 2013, it was announced by RCR that Dillon will drive the No. 3 Chevrolet in the NASCAR Sprint Cup Series with Cheerios and Dow Chemical Company as the primary sponsors. It would be the No. 3 car’s first Sprint Cup appearance in 13 years.

During 2014 Daytona 500 qualifying, Dillon clinched the pole position with a speed of 196.019 miles per hour (315.462 km/h). This was the fourth time that the No. 3 was on the pole for the Daytona 500.

After a long streak of bad finishes, Dillon rebounded with a top-five in the 2014 Coke Zero 400. Dillon ended the 2014 Cup season as one of only two drivers to have been running at the finish of every race during the season, the other being Jeff Gordon. He finished second in the rookie of the year standings to Kyle Larson.

Dillon competed part-time in the Xfinity Series in 2016, splitting the No. 2 of RCR with six other drivers. On the final lap of the Xfinity race in Fontana, Dillon battled with Kyle Busch, whose car had cut a left front tire, the two making contact as they approached the finish. Dillon’s car hit the turn four wall but was able to hang on for the win.

In the Xfinity Series, Dillon scored his 100th career top-10 Xfinity Series finish at California, which came as a fourth-place outing. He won the Xfinity race at Michigan in June after rain ended the race prematurely on lap 91. It was the first Xfinity Series win for Dillon since the August Bristol race in 2016, and the first for RCR’s Xfinity team since 2016 at Road America with Michael McDowell. Dillon also drove the No. 10 Chevrolet for Kaulig Racing at Indianapolis. This was the second time Dillon raced in an Xfinity event outside RCR. It is also the first time Kaulig Racing fielded two cars in a race.

Dillon became engaged to former NFL cheerleader Whitney Ward on August 9, 2016. Austin and Whitney were married December 9, 2017, at Childress Vineyards in Lexington, North Carolina. The two welcomed a son, Ace R.C Dillon, on June 14, 2020. Dillon attended High Point University on a part-time basis. He also played in the 2002 Little League World Series in South Williamsport, Pennsylvania.

When NASCAR returned to Martinsville for the first time in 2017, Dillon scored a season-best 5th place outing. This is his first top-5 and top-10 finish of 2017. At Texas, Dillon was unable to start the Cup race on the lead lap due to a broken track bar that put him 11 laps down. He finished 33rd.

In 2018, Dillon returned to the No. 3 car in the Cup Series. He drove this car to win the 2018 Daytona 500, after coming from a lap down within 20 laps to go to second on the final lap. He hit Aric Almirola in the right rear after Almirola tried blocking him, sending him into the outside wall. The win happened 20 years after Dale Earnhardt’s only Daytona 500 win, and it was also 17 years to the day of Earnhardt’s death. Despite the win and a guaranteed spot in the Playoffs, Dillon struggled to stay consistent throughout the season with two top-fives and five top-10 finishes. He was eliminated in the Round of 16 after hitting the outside wall twice at the Charlotte Roval race and finished the season 13th in points.

For 2019, Dillon will have Danny Stockman atop the pit box for the Cup Series. Stockman was Dillon’s crew chief in the Truck Series and Nationwide Series when he won his NASCAR Championship titles in 2011 and 2013.

On October 28, 2019, Danny Stockman announced he would step down as the crew chief of the No. 3 team at the end of the 2019 season. Justin Alexander returned as the No. 3 team’s crew chief in 2020 after having served that position in the 2017 and 2018 seasons.

In 2019, Dillon made a cameo in the movie Stuber alongside brother Ty and fellow driver Daniel Suárez.

Dillon guest starred in the television series SEAL Team’s Season 3, Episode 14 “Objects in Mirror”, as a NASCAR driving instructor for the protagonists. Filming took place at Auto Club Speedway in December 2019, while the episode premiered on March 11, 2020.