Moffitt was born August 7, 1992 in Grimes, Iowa. Moffitt is the youngest of four brothers.
Moffitt began his racing career at the age of 10 in kart racing competition. In 2007, Moffitt won the Harris Clash in the IMCA Sport Modified division. Following several years of competition in karts and dirt track racing, Moffitt began competing on asphalt in 2008, driving in the American Speed Association's Late Model North Series. He made his debut in NASCAR touring series competition in 2009, driving in the K&N Pro Series East – then known as the Camping World East Series – for a team owned by Andy Santerre; winning the pole in his first race at Greenville-Pickens Speedway, he became the youngest driver ever to do so in the series. He went on to score two wins over the course of the season, finishing the year third in series points. His first win, in the fourth race of the season at South Boston Speedway in May, made Moffitt the youngest driver ever to win in the series, and the youngest to win in any NASCAR touring series; the mark stood until the following season, when it was eclipsed by Darrell Wallace Jr. in March 2010.
For the 2010 season, Moffitt moved to Joe Gibbs Racing; he scored two wins during the year on his way to finishing second in points at the end of the season. In 2011, he drove as a development driver for Michael Waltrip Racing, winning three times over the course of the K&N Pro Series East season and finishing third in the series standings.
For 2012, Moffitt changed teams again, moving to Hattori Racing Enterprises, owned by former IndyCar and Truck Series driver Shigeaki Hattori. He won races at Richmond International Raceway and Central Pennsylvania Speedway, and led the series points after nine events of the fourteen-race season. A lack of sponsorship funding from HRE meant that for the final races of the season, starting at New Hampshire, the team associated with Michael Watlrip Racing to field Moffitt's car. He nearly won the 2012 K&N Pro Series East Championship, and was leading the final lap of the race and made contact with eventual winner Tyler Reddick, Moffitt crashed and lost the championship to Kyle Larson.
In August 2012, Moffitt made his debut in the NASCAR Nationwide Series, driving for RAB Racing at Iowa Speedway in the U.S. Cellular 250. He finished ninth in the event.
In 2013, Moffitt again Drove the K&N Pro Series East with Hattori Racing Enterprises. Moffit scored 5 top 5 finishes, and 10 top tens in 14 races and finished the season runner up in championship points.
Moffit was employed as a test driver for Michael Waltrip Racing. He made his Sprint Cup Series debut with the MWR-affiliated Identity Ventures Racing in the 2014 FedEx 400. Moffitt ran for IVR again at Michigan, Indianapolis, Bristol, Atlanta, Charlotte, Texas, and the season finale at Homestead.
In 2015, Moffitt drove in the No. 55 at Atlanta because MWR's regular driver Brian Vickers had off-season surgery to repair a patch placed over a hole in his heart. Moffitt played an impressive race, leading 1 lap, spending some time up in the top ten, and ultimately finished 8th for his first ever Sprint Cup top ten. Moffitt returned to the No. 55 at Fontana due to Vickers being sidelined with more blood clots, also declaring for Rookie of the Year at this time. He returned to the 55 at Martinsville and Texas.
Moffitt drove the No. 34 for Front Row Motorsports at Las Vegas and Phoenix because their regular driver David Ragan drove for Joe Gibbs Racing while JGR driver Kyle Busch is injured. It was announced that Moffitt will again replace Vickers for the next 5 races as he takes blood thinner medication and cannot race while using it. Moffitt was then replaced by Michael Waltrip for the Talladega race. Ragan became the driver of the 55 at the Kansas race, after Erik Jones took over the 18 ride. Later, in the week, Front Row Motorports announced that Moffitt would return to the team at Kansas and become the primary driver of the 34 for the rest of the 2015 season except at Sonoma, Watkins Glen, and Talladega.
Moffitt ended up running 31 races and won the 2015 NASCAR Cup Series Rookie of the Year honors over contenders Jeb Burton, Matt DiBenedetto, and Alex Kennedy.
On December 10, 2015, it was learned that Moffitt would not return to the No. 34 Ford Fusion in 2016, being replaced by 2015 NASCAR Xfinity Series champion Chris Buescher for the 2016 season.
In 2016, Moffitt was picked up by Red Horse Racing to race at Kentucky in the No. 11 Toyota Tundra. He started 2nd, and finished 31st after blowing an engine. Moffitt returned to the truck for Pocono, and Bristol finishing 3rd and 2nd respectively. On August 27, Moffitt won at Michigan in his fourth start in the No. 11 (and sixth career start) after passing both William Byron and Red Horse Racing teammate Timothy Peters on the final lap of the Career For Veterans 200. Moffitt also made a fifth start in the No. 11 at Canadian Tire Motorsport Park finishing 16th. Matt Tifft, the driver Moffitt was substituting for, returned to the 11 at Chicagoland Speedway after recovering from brain surgery earlier that year.
On February 17, 2017, it was announced that Moffitt would drive the first two races for Red Horse in 2017, behind the wheel of the No. 7. Moffitt was looking to run the full schedule, but on May 22, team owner Tom DeLoach announced the closure of Red Horse until sponsorship can be found, leaving Moffitt and teammate Timothy Peters without rides; at the time of the team's shutdown, Moffitt was tenth in points. In July, Moffitt joined GMS Racing's No. 96 team for his Xfinity Series return at Iowa Speedway. A month later, he signed with BK Racing to run the Cup races at Watkins Glen and Michigan in the team's No. 83 Camry.
(key) (Bold – Pole position awarded by qualifying time. Italics – Pole position earned by points standings or practice time. * – Most laps led.)
* Season still in progress
1 Ineligible for series points