The Chargers (2-2), outgained nearly 4 to 1 until late in the third quarter, made an apparent blowout close as Philip Rivers threw touchdown passes of 30 yards to Antonio Gates and 13 yards to Chris Chambers and Jacob Hester scored on a 41-yard fumble return, all in the fourth quarter.
Running back Mewelde Moore caught a touchdown pass and threw for another to Heath Miller on a goal-line play as the Steelers (2-2), beaten in the closing seconds by Chicago and Cincinnati the previous two weeks, built such a big lead that they withstood another bad fourth quarter. Jeff Reed finished it off with a 46-yard field goal in the final minute.
The first three plays showed how the AFC playoff rematch from a year ago would play out. Roethlisberger hit Santonio Holmes and Hines Ward for 15 yards each and rookie Mike Wallace for 35, and Mendenhall _ subbing for the injured Willie Parker _ needed three carries to cover the final 14 yards for his first career touchdown with only 3:11 gone.
Roethlisberger finished 25 of 32 for 326 yards and Ward had eight catches for 113 yards.
"We had to answer the bell," Roethlisberger said. "We did it all night long on offense. He did a great job running the ball. The offensive line did a great job opening up holes."
After San Diego went three and out, the Steelers put together a 75-yard drive that began with three Mendenhall runs but ended with Roethlisbeger finding Ward for 15, Holmes for 11 and Moore twice for 23 yards, with the touchdown covering 19 yards.
Mendenhall, a first-round draft pick whose rookie season ended a year ago in the fourth week of the season by a fractured shoulder, began the game with 103 yards rushing in his career. He had 100 yards by late in the third quarter and added a 32-yard run in the fourth, a week after he failed to get a single carry in Cincinnati because coach Mike Tomlin felt he didn't know his plays.
"I wanted people to see me and who I am," Mendenhall said. "This was a time that I was able to do that, play a full game for the first time in my career. The offensive line did a great job as a unit."
Not long after the Steelers made it 14-0 with 4:08 remaining in the first quarter, some NBC viewers might have been tempted to flip over to Pittsburgh-based medical drama "Three Rivers" on CBS _ if only because of the way the Chargers (2-2) were struggling alongside those very same rivers.
It hardly was Rivers' fault. With no running game to support him _ LaDainian Tomlinson had 15 yards on 7 carries _ Rivers (21 of 36, 254 yards, 3 TDs) almost didn't have a chance to get his offense going until it was too late after throwing for 739 yards in his previous two games. The Steelers held the ball for nearly 23 minutes in the first half to San Diego's 7 minutes.
San Diego was down 28-0 before a possession ended with anything but a punt, and the Chargers didn't advance inside the 20 until a drive that ended with Antonio Gates' 3-yard touchdown catch in the third.
San Diego cut it to 28-14 early in the fourth when the Steelers thought Stefan Logan was down on a punt return before the ball came out and watched as Hester ran into the end zone with no resistance. The Steelers unsuccessfully challenged the call.
The Chargers played in Heinz Field for the third time in 11 months _ as many appearances during that time as stadium tenant the University of Pittsburgh _ and lost for the third time. Nothing unusual there, as the Chargers are 0-14 in Pittsburgh during the regular season, although they have won there twice in the postseason.
Parker ran for 261 yards in those two games last season, but sat out wit an injured left big toe _ not that it made any difference the way Mendenhall ran against the NFL's 26th-ranked rushing defense.