Almirola began racing when he was eight years old, racing go-karts. At 14, he began racing nationally. He won the pole position in his debut in the World Karting Association race and finished fourth in the standings that year. Two years later, he moved up into modifieds and won several Rookie of the Year awards.
In 2002, Almirola moved to the NASCAR Sun Belt Weekly Racing Division and finished second in the Rookie of the Year standings. He followed that up with five pole positions in 2003. In 2004, he became one of the first drivers to participate in NASCAR's Drive for Diversity program. He ran the season at Ace Speedway, and won two races before finishing 11th in the points standings. He won five more races at the track in 2005, and made his Truck Series debut with Morgan-Dollar Motorsports and had two top-tens in four races.
For 2006, Almirola drove the #75 Spears Manufacturing Chevy for Spears Motorsports in the Craftsman Truck Series, as part of the JGR development program. He's also set to start seven Busch races this season, in the #19 Husqvarna Chevrolet, for Joe Gibbs Racing. He debuted the 19 at Richmond in May, starting 12th and finishing 32nd.
Almirola moved up to the Busch series full-time in 2007, driving the #18 and #20 Chevys for Joe Gibbs, driving the #18 and the #20 in seven races apiece. He won his second career pole award for the Orbitz 300 at Daytona, and the first he actually took the green flag after winning a pole, as his first pole at Milwaukee, he was forced to relieve the driving duties over to Denny Hamlin. He again won the pole at Milwaukee in 2007, but thought he was going to give up driving duties to Hamlin again. A late team decision allowed Almirola to start the race due to Hamlin not arriving before the start of the race. On lap 59 of the race, during a caution period, Hamlin took over for Almirola while Aric was running in the third place. Hamlin went on to win the race, but Almirola was credited with his first NASCAR Busch Series win because he was the driver who started the race. Almirola did not participate in team victory celebrations after the race as he had already left the track.
Following the incident at Milwaukee, Almirola departed Joe Gibbs Racing to join Dale Earnhardt Inc. following the sale of Ginn Racing. Almirola was named co-driver of the #8 United States Army Chevy in the 2008 Sprint Cup, sharing the ride with Mark Martin.