Williams yielded a two-run single to Gregg Zaun in the top of the 12th inning as the Blue Jays scored four times in that frame, ending the Orioles' winning streak at eight with a 6-2 victory.
Williams was charged with all four runs.
The crowd of 15,641, who withstood sub-50 degree temperatures, was the lowest in the 13-year history of Camden Yards.
They witnessed an Orioles club that appeared to be flat after an exciting sweep of the Devil Rays. The offense scratched together two runs in the sixth inning off tough Toronto left-hander Gustavo Chacin, and then was shut down, not collecting a hit for the remaining six innings.
That left the bullpen to try to hold down the Blue Jays, who lost slugger Carlos Delgado to the Florida Marlins in the offseason but still entered Monday's game ranked third in the American League in hitting.
With the score tied at 2, Williams began the 12th in inauspicious fashion, allowing consecutive singles to Vernon Wells and Corey Koskie. Right fielder David Newhan briefly mishandled Koskie's single and Wells motored to third base.
Against a drawn-in infield, Shea Hillenbrand hit a bouncer to shortstop Miguel Tejada, who fired home to get Wells, who headed back to third. Wells delayed the rundown just enough for Koskie to reach third and Hillenbrand to move to second.
With first base open, Williams was forced to intentionally walk a struggling Eric Hinske -- who was 0-for-5 with two strikeouts -- to face Zaun, who laced a single to center field.
"We still had a chance to get out of it," Mazzilli said. "I didn't feel [the rundown play] was big. If we get a ball hit right at somebody, it's a double play."
Mazzilli has formulated a bullpen rotation in the first month, and it has not included Rick Bauer, who has not appeared in a game in 12 days. John Parrish, who made his first appearance since April 20 on Monday, has also been mostly excluded.
"What's your other choice? You have one for me?" Mazzilli said to a reporter who asked about the bullpen use. "If I put Bauer in, then you're going to ask me how come I brought someone in who hasn't pitched in 10 days."
"There's really no excuse. I should be able to throw three games in a row, no problem," Williams said. "Looking at it now, I was pitching when I should have been throwing. I was just trying to get guys to chase when I should have been throwing strikes."
The Orioles tied the game in the sixth against crafty left-hander Gustavo Chacin, who was named American League Rookie of the Month earlier in the day. Sammy Sosa -- who trotted to first base on what he thought was ball four only to be called back to home plate -- laced a double off Koskie's glove at third base.
Javy Lopez followed with an RBI double to right-center field, and Jay Gibbons tied the game with a broken-bat single just over the outstretched glove of shortstop Russ Adams.
The Orioles appeared to have the momentum, but couldn't touch home plate against Toronto's bullpen. Blue Jays relievers combined for 4 2/3 no-hit innings and squashed the Orioles' biggest threat in the bottom of the 11th.
The Orioles loaded the bases in the 11th on a walk, an error and an intentional walk. With two outs, Larry Bigbie faced lefty Scott Schoeneweis with the bases loaded, but ended the threat by striking out.
"It's just that we had a couple of situations where the base hit that we've gotten all year would have won the game for us," Mazzilli said. "We just didn't get it."