McDowell started out his career at the age of three racing BMX bicycles. He would always win his division because there were not any other three- or four-year-olds to compete against. At the start of each race, someone would have to hold him in the starting gate because his feet did not reach the ground. Then, when McDowell was eight years old, he moved to karting with help from his mother Tracy, his father Bill, and his older brother Billy, who were all avid race fans. After ten years in karts, he would win not only the World Karting Association championship, but he would also take back to back undefeated International Kart Federation championships, along with 18 consecutive feature wins.
From karts McDowell moved into Formula Renault with World Speed Motorsports in 2002 and won the Infineon Raceway, Las Vegas, and Firebird International Raceway events in his rookies season. Moving up the ladder, Michael would move to the Star Mazda Series in 2003. He took victories at Sebring and Road America en route to a runner-up finish in the points. McDowell won seven races in 2004 en route to winning the series championship. McDowell would be picked up by Champ Car team Rocketsports for Surfer's Paradise and Mexico City, but decided to move on in 2005 to the Grand-Am Cup Series.
During his run for the Star Mazda championship, Michael would meet businessman Rob Finlay at the Bob Bondurant School of High Performance Driving, where McDowell was an instructor. McDowell and Finlay soon established a partnership, and McDowell would drive his first sports car, a Porsche 996 in the Grand-Am Cup Series for Finlay Motorsports. In 2005, Michael would stay at Finlay, but moved up to the Rolex Sports Car Series, joining veteran Memo Gidley in a BMW powered Riley sponsored by the Make A Wish Foundation. Together, they finished sixth in the overall points and took the team's first victory at Mexico City. McDowell made history by becoming the youngest driver to stand on podium for a Grand-Am race at Barber Motorsports Park, and became the first driver since Mario Andretti to compete in both an open wheel race and a sports car race in the same weekend. Michael would also drive the team's No. 60 BMW M3 in the Grand-Am Cup series to a podium finish at Daytona. McDowell would stick with Finlay for 2006, and was again partnered with Gidley. The duo would improve to fourth in points despite being winless. Michael would also drive a No. 15 Ford Mustang with owner Finlay, driving the car to a win at Barber. He would cap off his Grand-Am year with a second-place finish at Miller Motorsports Park.
McDowell returned to the series for the 2012 24 Hours of Daytona, driving for Michael Shank Racing and finishing third.
ARCA RE/MAX Series owner Eddie Sharp would put McDowell behind the wheel of his No. 2 Dodge for five races of the 2006 season, starting with the Governor's Cup 200 at the Milwaukee Mile, where he scored a 34th-place finish. McDowell finished in sixth place at Salem, fifth at Talladega, and fourth at Iowa. Michael would return to ESR to run the full schedule for Rookie of the Year honors in 2007. McDowell was involved in controversy at Toledo when his ESR teammate, Ken Butler III spun him late in the race. Butler would take his first win while McDowell would rally for a top ten finish. Michael got his first stock car win at Kentucky Speedway. He followed that up with wins at Pocono, Chicago, and the season finale at Toledo. Although Michael was in contention for the ARCA Re/Max Seroes championship all season, he was unable to pose a serious threat to Frank Kimmel for Kimmel's ninth championship. Michael would score a second-place finish in the points along with Rookie of the Year honors, as well as scoring nine pole positions during the season. As McDowell moved to NASCAR, his ESR ride was taken over by former Formula One driver Scott Speed.
McDowell would make his NASCAR debut in the Craftsman Truck Series. Driving the No. 17 for Darrell Waltrip Motorsports, McDowell would qualify 29th and finish 30th after a late wreck. McDowell moved to the Busch Series as a developmental driver for Nextel Cup Series team Michael Waltrip Racing. Driving the No. 00 Toyota, McDOwell drove at Texas, Phoenix, and Homestead, finishing respectively 20th, 14th, and 32nd.
McDowell was chosen to drive the No. 00 Cup Series car in 2008 after Dale Jarrett's retirement; David Reutimann replaced Jarrett in the No. 44 UPS car. McDowell started 34th and finished 26th in his Sprint Cup debut, the Goody's Cool Orange 500, after a flat tire near the end of the race. After the race, he was criticized by veteran driver Jeff Burton for having blocked Burton's way to leader Denny Hamlin near the end of the race (McDowell was racing for the Lucky Dog free pass, which Burton did not realize at the time of his comments). In early August 2008, McDowell was pulled from the No. 00 Toyota Camry Sprint Cup car in favor of veteran NASCAR driver Mike Skinner for three races. Skinner helped evaluate the team's progress, while trying to get the No. 00 into the top 35 in owner points, though McDowell returned to the ride at Richmond on September 6, 2008. McDowell was again pulled from the No. 00 Toyota Camry when he failed to qualify for the Camping World RV 400 at Kansas on September 28. McDowell's contract was not renewed by MWR for 2009, when Michael Waltrip decided not to retire and the team did not have sponsorship for another car.
In 2009, McDowell drove a part-time schedule for JTG Daugherty Racing in the NASCAR Nationwide Series. McDowell next attempted the April 2009 race at Talladega for Prism Motorsports in the Sprint Cup Series, as regular driver Dave Blaney had a family obligation. After JTG Daugherty Racing ran out of sponsorship support after the second race at Daytona International Speedway, it was announced that he would drive the next two races for MacDonald Motorsports. He also competed several races with Tommy Baldwin Racing in the No. 36 Toyota Camry. McDowell remained in the top 10 for most of the season. McDowell also competed in the Camping World Truck Series in 2009 for ThorSport Motorsports driving the No. 98 Chevy Silverado.
In 2010, McDowell started the season for Prism Motorsports. He drove the No. 55 car, then moved to the No. 46 team of Whitney Motorsports. In 2011, McDowell drove the No. 66 Toyota for HP Racing in the Sprint Cup Series. JGR also added the Bucyrus 200 at Road America to McDowell's schedule, leaving open the possibility of more races added. McDowell also drove one race for Joe Gibbs in the Sprint Cup Series at Texas after NASCAR parked Kyle Busch following his actions at a Truck Series event.
In 2012, McDowell ran the Joe Gibbs Racing No. 18 Nationwide Series car in select races, and competed in the Sprint Cup Series for Phil Parsons Racing (formerly HP Racing) full-time; the team secured full sponsorship for the first five races of the season. It formed a partnership with Whitney Motorsports and Mike Curb to field McDowell's car, which carried the No. 98.
In the 2013 Daytona 500, McDowell recorded a then-career high 9th-place finish. Later in the season in the Nationwide Series, McDowell won the pole for the inaugural Nationwide Children's Hospital 200 at Mid-Ohio. McDowell finished 2nd behind A. J. Allmendinger.
In October 2013, it was announced that McDowell would be moving to Leavine Family Racing's No. 95 Ford for the 2014 NASCAR Sprint Cup Series season. McDowell failed to qualify for the Daytona 500. McDowell had a Sprint Cup career-best seventh-place finish in the Coke Zero 400.
McDowell returned to the LFR No. 95 car for a 20-race schedule in 2015. The team merged with Circle Sport in 2016 and switched to Chevrolet, with McDowell splitting the schedule with Ty Dillon with sponsorship from Thrivent Financial, Cheerios, and WRL Contractors among others. McDowell was able to get a 10th place finish in the 2016 Coke Zero 400.
McDowell drove for Richard Childress Racing in the No. 2 Rheem Chevrolet in a one-race deal at Road America which got McDowell his first NASCAR victory. McDowell led the final 24 laps en route to the win.