Little is known of Beni's past: a Hungarian con artist and soldier, he was a member of the French Foreign Legion during the 1920s, supposedly a friend to fellow Legionnaire Rick O'Connell. On the orders of an overambitious commander, they were part of a garrison sent out of Libya and into the Egyptian desert to search for the lost city of Hamunaptra; though they found the city, they are quickly attacked by the Medjai defenders. Before the army can even reach the Legion's front lines, Beni flees the scene for the safety of the ruins - casually abandoning Rick to die at the hands of the Medjai by sealing the door behind him.
After making his way back to civilization, Beni leaves the Legion altogether and sets himself up as a fraudulent guide: as he eventually makes clear, his business thrives by charging exorbitant sums to lead travelers to Hamunaptra, only to abandon them in the middle of the desert before they can ask for their money back. However, he eventually ends up being hired by a party of American treasure-hunters who are smart enough to only pay half the agreed fee - the other half being paid upon their safe return to Cairo.
While accompanying the Americans on a ferry down the Nile, he bumps into Rick again; though Rick decides against killing him, he takes great delight in hurling Beni overboard. Unfortunately, this allows him to take the lion's share of horses and provisions when the Medjai attack the barge and force the adventuring parties overboard. Fortunately, Beni's party end up on the wrong side of the river, allowing Rick and the others a headstart to Hamunaptra.
Later, when the two parties finally arrive at the lost city, the American treasure-hunters are the first to uncover a find of real value: a sealed casket containing Anck-su-namun's canopic jars and the Book of the Dead. However, the curse inscribed on the casket spooks Beni, and he flees before the box can be opened - unwittingly saving his life in the process; when Evelyn reads from the book and accidentally resurrects Imhotep, Beni is one of the few members of the party not to be targeted by the mummy for assimilation. However, after abandoning Mr Burns in the tunnels under Hamunaptra, he is soon confronted by Imhotep (now sporting Burns' stolen eyeballs and tongue); in a panic, Beni begins reciting prayers in a number of different languages in the hope that one of them might be able to save his life. Recognizing Hebrew ("the language of the slaves") among one of the prayers, Imhotep spares Beni's life in exchange for his services as a translator, offering a handful of golden funerary treasure to sweeten the bargain.
From hereon, Beni serves as Imhotep's minion, not caring that it means the death of his supposed friends and the entire world being doomed if Imhotep wins. Over the course of the movie, he helps the mummy track down and kill the cursed Americans one by one, even mocking the horribly-mutilated Burns as he pleads for his life; for good measure, he also helps search for the Book of the Dead - though this once again brings him within punching distance of Rick O'Connell.
After helping to capture Evelyn and deliver her to Hamunaptra, Beni is left unsupervised while Imhotep begins the ritual to resurrect Anck-su-namun; recognizing an opportunity to profit further before fleeing, he gathers up as much of the city's treasure as he can carry and loads it into the saddle bags of his camels. However, his greed is ultimately his undoing: not satisfied by the small fortune already prepared, he returns to the treasure vaults for more - only to accidentally trigger an ancient booby trap in the process. Trapped in the collapsing tunnels under Hamunaptra, he is sealed inside the treasure vault as flesh-eating scarabs fill the room, presumably eating him alive. His body was never found.
In a final twist, Rick and Evelyn end up riding off into the sunset with Beni's camels, unwittingly claiming all the wealth that he had stashed in the process.
Beni is an arrogant, avaricious, cowardly, lying, sycophantic, and thoroughly unpleasant little man who shows little loyalty to any side in any given conflict, casually betraying even those he considers his friends in order to preserve his own life and fortunes: the only guarantee of his services to the Americans is the promise of a larger fee, and the only the guarantee of his service to Imhotep is the threat of a horrible death - which only works as long as the mummy is around to keep an eye on him.
Petty, vicious, and cutthroat, Beni is more than happy to gloat and torment others while in a position of power, gladly rubbing Mr. Burns's imminent death in his face; if his position shows any sign of weakness, however, he will flee. For example, when first confronted by Rick on the ferry, he responds with open toadying and pleas for mercy, but later, when armed and with the Americans at his back, he appears far more confident. Similarly, during his first encounter with Imhotep, he doesn't even consider running or fighting, instead immediately resorting to shamelessly begging for his life.
In spite of his selfish and cowardly nature, he does seem to have something akin to a guilty conscience later on, as he is seen looking away when Imhotep devours one of his targets; of course, this may only be due to a weak stomach, given that he's also seen hastily leaving Mr. Burn's room in a state of revulsion just as Imhotep begins assimilating his organs.
Ironically, despite accusing O'Connell of having "more balls than brains", Beni is nowhere near as clever as he thinks: though well-versed in numerous languages and customs, his knowledge is shallow and patchy in places, leading Evy to irritably correct his translation at one point. More often than not, his habit of foolishly goading and mocking rivals gets him beaten up, and his greed ends up dragging him back to Hamunaptra despite the danger. In a final display of greed and stupidity, Beni's unwillingness to part with his newest sack of treasure only slows him down, leaving him trapped in Hamunaptra and doomed to be devoured by scarabs.
Though he didn't appear in The Mummy: The Animated Series, which is based on the first two films that is set on its own continuity, Beni was heavily referenced on Colin Weasler, whom is regarded by fans as Beni's animated version.
Baltus Hafez can be considered his successor in the sequel. Both characters die nasty deaths, are cowardly, are supportive of Imhotep, and are disliked by the audience. However, Beni did not deliberately awaken Imhotep as Hafez did, though he enjoys watching Imhotep kill Mr, Burns.
In an ironic twist of fate, Beni technically played a posthumous role in the defeat of the Scorpion King in The Mummy Returns, as Jonathan mentions that the Scepter of Osiris was part of the loot Beni packed onto the camels in the first film. In other words, Beni's greed saved the world.