A Cory Gibbs goal against the run of play gave the home team the lead on 13 minutes before second-half strikes from Wayne Rooney, Rafael and Nani handed Sir Alex's men a hard-fought victory.
Indeed, Chicago were the better team for long periods and but for some wasteful finishing would have won this content. But again, as in Seattle three days earlier, the Reds proved clinical in front of goal and reaped the rewards.
Played in baking heat, in front of the largest ever crowd at Soldier Field for a game of club football, United were given a stern test by a Chicago side lying third from bottom in the MLS Eastern Conference.
Sir Alex handed first pre-season starts to the five players who joined the tour in Seattle, including summer signings David de Gea and Phil Jones. They were part of a young back five, with only Patrice Evra and Fabio old enough to leally drink in the US.
The Reds started well and carved out the game's first chance after five minutes. Danny Welbeck produced a sublime piece of skill to outfox three defenders on the halfway line before slipping a pass through the Fire defence for Dimitar Berbatov. The Bulgarian took his shot first time but Chicago stopper Sean Johnson did well to get down to his right and beat the ball away.
Given United's bright start, it was a surprise when the home side took the lead. Fire defender Gibbs outjumped Jones to reach a long Marco Pappa free-kick and head inside de Gea's right-hand post. There was little the Spanish