Robby Gordon pipped Mansell at the finish line, edging the British driver out of fourth place by half a car length.
For Penske it was a clean sweep of the top three places for the second time in three races, with the Brazilian Emerson Fittipaldi second and Canada's Paul Tracy third over 102 laps of the 1.95-mile (3.15km) road circuit in this Oregon city. Penske now needs one more triumph to equal the IndyCar record for consecutive wins by an owner set 20 years ago by George Bignotti.
Unser used his pole position to surge into the first turn ahead of Mansell, who is expected to be named among the Williams-Renault team's drivers for the French Grand Prix on 3 July at a press conference at Brand Hatch tomorrow. The American extended his lead to more than nine seconds for much of the race. Fittipaldi and Tracy moved past Mansell at the midway point but struggled to catch Unser.
Unser's winning speed of 107.777mph (172.443kph) was a race record, breaking the mark of 105.218mph (168.338) set by Andretti in 1992.
Unser has 105 championship points after seven of 16 races, with Fittipaldi second (86), Gordon third (64), Michael Andretti fourth (59) and the reigning champion Mansell fifth (56).
On the subject of Mansell and Formula One, Carl Haas, the joint owner of Mansell's IndyCar team, admitted that there was a 'high probability' that he would be competing in France, but added: 'Nothing has been settled yet.'