U.S. coach Gregg Berhalter changed five starters from the previous qualifier, a 1-1 draw at Jamaica in November, with Christian Pulisic, Weston McKennie and Tyler Adams all starting together for the first time in almost three years.
The U.S. continued its habit of slow starts in this World Cup qualifying cycle and, despite having the better of possession, failed to find the back of the net as the first half ended 0-0.
Robinson wasted no time breaking the deadlock early in the second half, pouncing on a loose ball at the back post in the 52nd minute and hammering a low volley past El Salvador goalkeeper Mario Gonzalez to open the scoring.
Matt Turner took over in goal from Zack Steffen, who remained in England because of a bad back, and didn't have to make a save. Turner played following reports he will move next summer from New England to Arsenal.
The Americans' 15th straight home win over El Salvador since 1989 didn't come easily. They outshot El Salvador 8-3 in the first half but put none on target. But the offense followed its pattern: The U.S. has scored 11 of its 13 goals in the second half.
The U.S. has two more cold-weather matchups in the next week and can move to the verge of a berth with victories on Sunday against Canada in Hamilton, Ontario, and Wednesday against Honduras at St. Paul, Minnesota. The final three qualifiers are in March.
The top three nations qualify, and the fourth-place team advances to a playoff against the Oceania champion, likely New Zealand. Winning home games is a key, and the Americans have won four of five matches on U.S. soil, stumbling only in a draw against Canada in September.
Information from The Associated Press was used in this report.