First-half goals from Eddie Johnson and Luis Silva inspired high-flying United to victory at Buck Shaw Stadium - the capital club's second successive win since the MLS resumed at the end of June following a FIFA World Cup break.
United States international striker Chris Wondolowski pulled a goal back six minutes before the half-time interval but it proved to be nothing more than a consolation as Western Conference cellar-dwellers the Earthquakes slumped to a fourth consecutive defeat.
Ben Olsen's United have collected 31 points after 18 rounds to be five points clear of Sporting KC, who are due to play the Montreal Impact on Saturday.
The Earthquakes have 16 points from 16 matches in the West - four less than second-bottom Chivas USA - though they have a game in hand.
United had won four of their past five away matches in MLS heading into Friday's clash and they put themselves on track to maintain that impressive record courtesy of a 12th-minute penalty.
After Brandon Barklage handled the ball inside the penalty area, Johnson stepped up to convert the spot-kick - just his second goal since arriving from the Seattle Sounders in December.
The visitors doubled their advantage 13 minutes later, when Silva found the back of the net for his fourth goal of the season.
United had chances to put the game beyond doubt but they were pegged back in the 39th minute following Wondolowski's fortuitous goal that took a wicked deflection from the edge of the box.
The Earthquakes pressed forward in search of a second-half equaliser but United held firm to secure maximum points away from home
