It was the seventh 100-yard game of the season for McAllister, who came alive after running for just 31 yards on 10 attempts in the first half.
"There were some limitations, but I still know what I can do," McAllister said. "Defenses are going to try to play me a certain way and you just have to be smart."
"His ankle was bothering him all week and didn't practice much this week," New Orleans Coach Jim Haslett. "He said at halftime he could get rolling because our offensive line was knocking people off the line of scrimmage."
Baltimore's Jeff Blake passed for 316 yards and two touchdowns in his first game against his former team, but the Ravens were their own worst enemy.
New Orleans blocked two punts and forced five turnovers. The Saints, who last blocked two punts in a game in 1972, converted those mistakes into 17 points.
"You don't win in this league when you turn the ball over and you certainly don't beat a team the caliber of the New Orleans Saints when you turn it over," Baltimore Coach Brian Billick said.
With a favorable remaining schedule, the Saints (9-4) remained one game behind Tampa Bay in the competitive NFC South. New Orleans is a half game ahead of Atlanta in the division. The Saints close with contests against Minnesota, Cincinnati and Carolina.
"It's a big game, you have to win in December to set yourself up for the playoffs and we were scared to death of this team with Jeff Blake as the quarterback," said Jake Delhomme, who took over for an injured Aaron Brooks in the second half.
The loss dealt Baltimore's playoff hopes a severe blow. The Ravens (6-7) failed to take advantage of Pittsburgh's surprising loss to the expansion Houston Texans and remained 1 1/2 games behind the Steelers. They also fell a game behind Cleveland in the AFC North.
Trailing, 27-7, the Ravens got a spark when running back Jamal Lewis turned a short pass into a 77-yard touchdown with 8:41 left in the third quarter.
Lewis also rushed for 48 yards on 17 carries, but caught fuor passes for 108 yards, his first career 100-yard receiving game.
Early in the fourth period, Baltimore reached the New Orleans eight, but had to settle for Matt Stover's 26-yard field goal with 12:54 remaining.
Delhomme, who replaced starting quarterback Aaron Brooks early in the second half, converted two keys plays on the ensuing drive in which the Saints took more than six minutes off the clock.
He avoided a sack and scrambled for a seven-yard gain on second and seven from the Saints' 20, then found Jake Reed for a 54-yard completion three plays later.
The Saints settled for John Carney's 28-yard field goal, but put away the contest just under four minutes later on McAllister's third touchdown, a six-yard run with 2:57 to play.
Brooks left with the same deltoid injury that bothered him last week against Tampa Bay.
"I feel better than I did after last week's game and that's a positive," Brooks said. "It's the same injury, a deep bruise in my deltoid muscle. It's something that I have to work out."
Brooks was ineffective, completing only nine of 25 passes for 123 yards. However, he had no turnovers. And with the defense repeatedly making big plays, Brooks did not need to excel.
Defensive end Willie Whitehead recovered Blake's fumble at the Baltimore 22, setting up Carney's 34-yard field goal with 8:46 left in the first quarter.
On the Ravens' next possession, New Orleans' Fakhir Brown recovered a blocked punt at the Ravens 39, which led to Brooks' 28-yard pass to rookie Donte' Stallworth that made it 10-0.
Blake's 43-yard pass to tight end Todd Heap gave the Ravens a first and goal at the one and set up Lewis' dive up the top with 2:22 left in the first period.
But the Saints scored 10 unanswered points in the second quarter on Carney's 28-yard field goal and McAllister's one-yard touchdown run with 33 seconds left in the half.
Lewis fumbled on the Ravens' second possession of the second half and tackle Kennny Smith recovered for the Saints at the Baltimore 22. Five plays later, McAllister went three yards off right tackle to make it 27-7 with 9:06 to go in the third quarter.
The Ravens got a late score on Blake's six-yard touchdown pass to Terry Jones.