DreamWorks' distribution president, Jim Tharp, said bad reviews may have hurt ticket sales in some bigger East Coast towns.
Pitt plays a member of the mob sent on an errand to retrieve a priceless pistol, while Roberts plays his long-suffering girlfriend, who is keen for him to find another profession.
Hannibal, the blood-curdling sequel to Silence of the Lambs, slipped to third place with $10.1m (£6.8m) after children's movie See Spot Run nudged ahead with $10.2m (£6.9m).
But Ridley Scott's Hannibal, starring Anthony Hopkins and Julianne Moore, has still managed to rake in $142.8m (£97m) since its release 25 days ago.
Both The Mexican and See Spot run had bad critics' reviews, but the pulling power of Pitt and Roberts was enough to draw the crowds.
DreamWorks' distribution president, Jim Tharp, said bad reviews may have hurt ticket sales in some bigger East Coast towns.
See Spot Run's success may be due in part to the fact that it does not currently have any rivals in the children's film market.
The movie, which cost about $15m (£10.1m) to make, pairs comic actor David Arquette with a canine FBI agent, and its opening was described by Warner Bros distribution president Dan Fellman as "very, very good".
The Chris Rock comedy Down to Earth dropped two places to No. 4 with $8m ($5.4m) and a 17-day total of $44.1m (£29.9m).
Awards boost
And Oscar-nominated Chinese-language martial arts romance Crouching Tiger, Hidden Dragon is still going strong, with $4.9m (£3.3m) million and an 87-day total of $88.7m (£60.2m).
The film has been boosted by several major awards and its roll out to an increasing number of cinema screens.
But it was not such a rosy picture for two releases that opened last weekend - 3000 Miles to Graceland and Monkeybone.
Graceland, a casino heist caper starring Kurt Russell and Kevin Costner, fell six places to number nine with $3.1m, (£2.1m) losing 57% of its audience and a total of $12.2m (£8.2m).
Monkeybone, a comedy starring Brendan Fraser and featuring a myriad of special effects, made just over $1m (£0.67m), 62% less than last week when it opened at number 11.
The film, which cost about $70m (£47.5m) to make, has grossed $4.3m(£2.9m) so far.