Fulham defender Robinson lashed home in the 52nd minute after Timothy Weah's good lead-up work in the decisive moment, as USA moved up to 18 points from nine games.
USA temporarily moved into top spot in the CONCACAF qualifying standings, with Canada due to take on Honduras later.
Jesus Ferreira, starting in a World Cup qualifier for the first time, burnt two first-half chances, before Robinson broke the deadlock early in the second half as USA stamped their superiority.
The host side pressed for a second goal, with El Salvador goalkeeper Mario Gonzalez acrobatically denying Valencia midfielder Yunus Musah's poke.
El Salvador's best chance came in the 85th minute as Joaquin Rivas glanced Bryan Tamacas' cross wide, while USA fluffed opportunities with Weston McKennie and Gyasi Zardes headers.