The fourth instalment in the Shrek franchise was the top film at the North American box office over the weekend, taking $71.3m (£43.9m) on its first three days on release.
Yet Shrek Forever After did not perform as well as its two predecessors, Shrek 2 and Shrek the Third, both of which made more than $100m (£69m) in their debut weekends.
A spokeswoman for DreamWorks Animation admitted its takings were "a little lower than we anticipated".
Featuring the voices of Mike Myers, Eddie Murphy and Cameron Diaz, the fourth Shrek film is the first in the series to be shown in 3D.
Its chart-topping debut ended Iron Man 2's two-week reign as box office champion in the US and Canada.
The comic book sequel dropped down a place to two with third weekend takings of £26.6m (£18.3m), taking its overall domestic tally to more than $251.2m (£173.9m).
Shrek Forever After finds its eponymous ogre experiencing a mid-life crisis that leads him to strike a bargain with crafty Rumpelstiltskin.
According to Anne Globe, DreamWorks Animation's head of worldwide marketing, the studio was "kind of in uncharted territory".
"It's not where you start, it's where you finish," she said of the film, which will be released in the UK on 2 July.
Ridley Scott's Robin Hood film also fell one place to three with a second weekend haul of £18.7m (£12.9m).
Starring Russell Crowe, the film - believed to have cost at least $155m (£107.4m) to make - has underperformed at US and Canada cinemas.
Another film that failed to meet expectations was MacGruber, a comic spoof on the 1980s TV series MacGyver starring Will Forte.
Based on a sketch featured on Saturday Night Live, the film opened in sixth place with weekend takings of $4.1m (£2.8m).