[5] Moving up to Legends cars at age 12, Blaney won the Lowe's Motor Speedway Summer Shootout and the Young Lions Winter Heat Points Championship, as well as three divisions of the Carolina Fall Nationals in quarter midgets.

In July 2012, Blaney announced that he had signed a contract to drive for Team Penske a minimum of three races in the 2012 Nationwide Series season, starting at Iowa Speedway in August.

However, his year's turning point was at Dover in late May 2014, when he came up short to Kyle Busch who beat him by 0.5 (one car length) seconds for the win.

Blaney won his second career Nationwide race in August 2014 at Bristol Motor Speedway, beating Kyle Busch in a green-white-checkered finish.

The next week, Blaney won his first Truck race of 2014 spectacularly at Ron Fellows' own Canadian Tire Motorsports Park raceway, edging German Quiroga by 0.49 seconds in a photo-finish.

He nearly won at Indianapolis Motor Speedway in the Xfinity race, finishing second to Kyle Busch after being passed on the final lap.

Before the race at Auto Club Speedway, Blaney and close friend Bubba Wallace drove together from Phoenix to Fontana and stole the headlines one evening when they took over NASCAR's Snapchat account and filmed themselves mimicking drivers such as Carl Edwards, Danica Patrick, and Paul Menard.

[27] Blaney picked up more top tens at Phoenix, Talladega, Dover, and Pocono for a successful first half of the season with his best finish being a fifth at Kansas.

During the Daytona 500, Blaney, despite being caught up in an early wreck, charged from 5th on the final lap to place second to Kurt Busch.

[32] Blaney then scored consecutive top-ten finishes at Martinsville and Texas, but despite winning the pole for the Round of 8 finale at Phoenix, he faded outside of the top ten by the end of stage one.

He rebounded to secure eighth by the end of stage two, but could not improve his standing on the track, ultimately finishing the race in 17th and being eliminated from the playoffs.

Returning to Daytona in July was not kind to Blaney, as he could not replicate his success from the 500, being caught up in The Big One and finishing in last place.

[42] Blaney won the Talladega fall race in a photo finish of 0.007 seconds over Ryan Newman and advanced into the Round of 8, despite an early spin onto pit road.

[44] On the final lap of the 2020 Daytona 500, Blaney gave a push to leader Ryan Newman, sending him into the outside wall and into a violent crash.

Blaney later stated that the incident had taken a tough toll on him mentally, and that speaking with Newman on the phone after his release from the hospital was a key factor to helping him become at peace with it.

Blaney led the most laps at Texas and finished fourth at Michigan, putting up a strong coalition of runs going into the playoffs.

Blaney made the playoffs with his win at Talladega, but was eliminated in the Round of 16 due to mediocre finishes at Richmond and the Bristol Night Race.

[50][51] For the first time in his career, Blaney had back-to-back wins after he won the last race of the regular season at Daytona International Speedway the following week.

At Bristol in April, Blaney would be his Cup teammate, Joey Logano's spotter for the Weather Guard Truck Race on Dirt.

He led the most laps at the GEICO 500 at Talladega, but finished second to Kyle Busch on an overtime restart after a bad block from Bubba Wallace.

[60] A poor performance the next week at Sonoma was followed by a scary moment at Nashville; Blaney had a violent impact into the inside wall of Turn 1, leaving him suffering with concussion-like symptoms.

In the regular-season finale at Daytona, Blaney charged to the front late in the second stage but was involved in a hard crash from the lead after Ty Gibbs was turned by Christopher Bell, causing The Big One.

[67] He finished sixth at Las Vegas, but was disqualified after post-race inspection discovered a left-front damper that did not meet specifications; as a result, Blaney dropped to last-place and the bottom of the Round of 8 cutoff line.

He traded the lead back and forth with Denny Hamlin for most of the race, but won stage two and held off a late-charging Aric Almirola to get his third win of the season and lock himself into the Championship 4.

[73] Blaney once again fell short of a victory at Gateway after running out of fuel on the last lap; his teammate Austin Cindric ended up winning the race.

[75] During the playoffs, Blaney advanced to the Round of 8, where after a narrow loss to Tyler Reddick at Homestead, he would win at Martinsville to make the Championship 4 for a second year in a row.

[76] Blaney ran up front for the bulk of the season finale race at Phoenix, though would be unable to outrun his Team Penske teammate Joey Logano, who would go on to win the 2024 Cup Series Championship At the beginning of the 2017 season, Blaney began a NASCAR-supported podcast featuring former Miss Sprint Cup and current Motor Racing Network personality Kim Coon and NASCAR.com analyst Chuck Bush, called the Glass Case of Emotion.

[78] In June 2017, Blaney became a member of Fox NASCAR's Cup driver-only broadcast team for the Xfinity Series race at Pocono Raceway, working as a pit reporter alongside Erik Jones and Ricky Stenhouse Jr.[79] He also interviewed race-winner Brad Keselowski; the next day, the roles were reversed as he won his first Cup race with Keselowski, still in his firesuit, interviewing him.

[87] In 2019, Blaney had a cameo appearance as himself in the Netflix's television series The Crew alongside Austin Dillon and Cole Custer.

He is of Irish descent and is close friends with fellow drivers Bubba Wallace, Erik Jones, and Chase Elliott.