Frenchwoman Camille Abily gave the Sol the lead in the 35th minute when shot into the roof of the net after Marta headed Shannon Boxx's cross from the left wing to the far post.
Boxx doubled the Sol's advantage in the 50th minute, drilling home a cross from Han Duan.
The Sol's possession game was the difference.
"We got forward and created great space," said Boxx, who had scored on Monday in the USA's 4-0 win over Canada in Toronto. "It was perfect and made my goal easy."
The shutout was Canadian Karina Leblanc's seventh of the season.
Washington, the only other team with a winning record (3-0-2), moved into second place with a 3-1 win over FC Gold Pride.
An own goal by Kristen Graczyk set the tone for what was a long afternoon for the Pride, which trailed, 3-0, within 32 minutes and had Carrie Dew sent off late in the game for being issued two yellow cards. The ejection was only the second of the WPS season.
The Freedom struck again in the 22nd minute on Lisa DeVanna's goal, and defender Kat Whitehill made it 3-0 when her shot defected off the back of the Pride's ever-steady center back Rachel Buehler and found its way into the net.
"This was a difficult loss especially when we gave two goals up for them," said FC Gold Pride coach Albertin Montoya. "You can't give anyone two goals in this league, it's very difficult to come back from that. I'm very proud of our team, they came out in the second half and played really well, created plenty of chances to get back in the game and scored a very good goal."
Canadian striker Christine Sinclair scored her third goal of the season in the 56th minute.
Sky Blue FC won only its second game when it edged Boston, 2-1, to join Chicago and Saint Louis in a tie for fifth place with eight points.
May 30 in Carson, Calif.
Los Angeles 2 St. Louis 0. Goals: Abily (Boxx, Marta) 35, Boxx (Duan) 50.
Los Angeles - LeBlanc, Makoski, Falk, Franko, Cox, Abily, Boxx, Bock (Zerboni, 86), Miyama, Duan (Larkin, 87), Marta.
St. Louis - Solo, Larsson, McNeill, Ellertson, Woznuk, Chalupny, Francielle (Welsh, 75), Fletcher (Weber, 46), Cross (Stoia, 65), Hanks, Aluko.
Referee: Kari Seitz.
May 30 in Germantown, Md.
Washington 3 FC Gold Pride 1. Goals: Own goal 16, De Vanna 22, Whitehill 32; Sinclair 57.
Washington - McLeod, Gilbeau, Sauerbrunn, Singer, Whitehill, Bompastor, Lindsey, Sawa, De Vanna (Moros, 69), Long (Lohman, 76), Wambach.
FC Gold Pride - Barnhart, Buehler, Dew, Robinson, Adriane (Arakawa, 77), DiMartino, Formiga, Graczyk (Abegg, 54), Osborne, Milbrett (Weimer, 77), Sinclair.
Yellow Cards: Washington - Bompastor 44; FC Gold Pride - Formiga 45, Dew 65.
Red Card: FC Gold Pride - Dew 83.
Referee: Sandra Serafini.
May 31 in Piscataway, N.J.
Sky Blue FC 2 Boston 1. Goals: Kai (Schnur) 4, White (Schnur) 29; Smith 12.
Sky Blue FC - Branam, Dowling, Asante, Rampone, Schnur, O'Reilly, Averbuch, Rosana (Parker, 68), McCallum, Kai (Sitch, 76), White.
Boston - Luckenbill; Mitts, LePeilbet, Weber, Scot, Bishop (Chapman, 76), Hucles, Smith (Tomecka, 45+), Lilly, Schmedes, Rodriguez (Nobis, 46).
Yellow Cards: Sky Blue FC - Parker 90+; Boston - Mitts 48.
Referee: Bryan Roslund.
TEAM W L T GF GA P
Los Angeles 6 1 2 11 2 20
Washington 3 2 3 15 15 12
Boston 3 3 2 10 11 11
FC Gold Pride 3 5 1 9 13 10
Chicago 2 3 2 6 6 8
St. Louis 2 3 2 6 8 8
Sky Blue FC 2 4 2 6 8 8
Kelly Smith (Boston) 5
Marta (Los Angeles) 4
Christine Sinclair (FC Gold Pride) 3
Lisa De Vanna (Washington) 3
Camille Abily (Los Angeles) 3
Wednesday, June 3
Los Angeles vs. Chicago, 10:30 pm ET
Sunday, June 7
Saint Louis vs. Sky Blue FC, 4 pm ET
FC Gold Pride vs. Chicago, 8 pm ET
Los Angeles vs. Washington, 6 pm ET (FSC)