Carolina, North Georgia Speedway in Chatsworth, Georgia, and Nashville Superspeedway. Cassill was signed by Hendrick Motorsports in December 2006. In 2007, Cassill tested the team's Car of Tomorrow car at Lakeland Speedway and Greenville-Pickens Speedway. Along with his driving duties, Cassill has worked in the team's research and development program and drove during practices for the Hendrick pit crews. Cassill made his Nationwide Series debut at Gateway International Speedway in July 2007 after turning 18, as NASCAR requires national series drivers to be at least 18 years of age (regional series drivers can be 16). Cassill finished 32nd in his debut. He was in contention to score his first top-10 finish at Memphis before another driver spun him out on the final lap. In his six starts that year, his best finish was 18th at Dover. In 2008, Cassill drove the No. 5 National Guard Chevrolet in 16 Nationwide series races for JR Motorsports and the No. 4 Phoenix Racing Chevrolet in the 3 road course races. In Cassill's first start of the season, at Nashville, he started 22nd and finished 19th, two laps down. He earned his first top-10 finish in the June race at Nashville with a 9th-place finish. He won his first pole at Loudon, but had to start at the back of the field due to an engine change. He quickly moved his way up in the race but was wrecked by Bobby Hamilton Jr., resulting in a 34th-place finish. He was involved in another incident with Hamilton Jr. at Memphis which resulted in an altercation following the race. In 19 starts, Cassill earned 5 top-10 finishes and won Rookie of the Year honors. He also drove a limited schedule in the Truck Series for Randy Moss Motorsports; garnering 3 top-10 finishes. Cassill made his lone 2009 start on October 24 at Memphis; driving Phoenix Racing's No. 1 Miccosukee Chevrolet to a 10th-place finish. In 2010, he drove the No. 98 truck for Thorsport Racing in the NextEra Energy Resources 250 at Daytona, but was caught in an accident on the first lap of the race. He made 6 starts in the Nationwide Series: 3 in the No. 7 Chevrolet for JR Motorsports and 3 in the No. 09 Ford for RAB Racing. Cassill made his Cup Series debut at Michigan; driving for veteran team owner James Finch. He finished 38th in his Cup debut. He