Johnson took over from Roush at the halfway point of the 87-lap, two-and-a-half hour race on the 2.25-mile Thunderbolt Raceway course, and - after coming out of the pits second - moved around Joe Koenig on the inside of Turn 1 on Lap 49. He engaged in a spirited battle for several laps with second-place finisher Matt Plumb, but Johnson was never headed after he put the No. 61 ROUSH Performance Products Ford Mustang GT up front.
The winning margin of victory was 1.384 seconds, the seventh race this season with a margin of victory under two seconds.
"It was tough for my stint," Johnson said. "I had two fast cars - with fast teams and fast drivers - behind me. I managed that as best as I could. I had to work hard to keep the tires under us for the end."
Roush, who earlier Saturday garnered a track qualifying record of 1:30.109, led the opening 44 laps, which featured four of the race's five cautions. Johnson led the final 39 laps.
Plumb and new co-driver Nick Longhi finished second in the No. 13 RumBum.com BMW M3, which fell several positions early as the team opted for a two-pit stop strategy. They were joined on the podium by Joey Hand and Michael Marsal in the No. 97 Turner Motorsport BMW M3.
Fourth were Terry Borcheller and Joey Hand in the No. 45 Fall-Line/Stable One BMW M3, while Barry Waddell and second-place starter David Russell garnered fifth in the No. 99 H&S Tools BMW M3.
Despite falling to 23rd during an early race spin, Charles Espenlaub and Charlie Putman rebounded to finish eighth in the No. 48 Fall-Line/Sparco BMW M3. The pair maintained the championship lead in the standings, which now stands at 10 points (229-219) over Hand and Marsal. Johnson and Roush are 15 points behind the leaders.
In the Street Tuner class, Owen Trinkler and Randy Smalley took the No. 198 Cruise America/4 Winds RV Mini Cooper S to its first-ever victory in the series. Previously, the best finish for a Mini Cooper was third twice.
Trinkler and Smalley rebuilt a badly-burned car from an earlier race this season at their new RSR Motorsports shop in Florida, and the race was the first for the new Mini. Trinkler traded the lead twice with Ryan Eversley in the No. 75 JC Concrete/HPD Honda Civic Si before distancing himself from the field. The margin of victory in the ST race was 16.071 seconds over James Clay in the No. 80 RAYS Engineering/Performance Friction BMW 328i.
But the triumph didn't come without a little warning.
"We were a little short on fuel - we might have made it with that yellow - but we've had such bad luck that we just didn't want to risk it," Trinkler said. "And it was the right call."
The victory was the second in the series for Trinkler - and his first since 2003 - and Smalley's first ever.
Clay and David White finished second, while polesitter Martin Jensen and Paul Gerrard, who combined to lead 53 of 86 laps, were third in the No. 18 i2i Capital/RRT Racing BMW 328i. Eversley and Zach Lutz finished fourth, while points co-leaders Seth Thomas and Bill Heumann were fifth in the No. 81 RAYS Engineering/Performance Friction BMW 328i. The No. 81 started from the rear of the field after an engine change Saturday morning.
Additionally, Thomas and Heumann stretched their championship lead to 13 points (238-225) over Lawson Aschenbach and David Thilenius. Not long after Aschenbach took over from Thilenius, who ran second during his entire stint, the No. 74 Skunk2/HPD Honda Civic Si experienced transmission troubles. Aschenbach and Thilenius finished 17th, their worst finish of 2010.
The ninth of 10 rounds on the 2010 circuit comes Aug. 13-14 in the annual Grand Prix de Trois-Rivières in Trois-Rivières, Quebec.