The Malfoys have generational wealth (i.e. they inherit). It's only in the movie Chamber of Secrets that Lucius is represented as having a job at the Ministry; this is not reflected in the book. He is on the Hogwarts Board of Governors for at least two years SS/PS and CoS, but I suspect that is an unpaid position.
According to Forbes, Lucius Malfoy is number 14 on the list of wealthiest fictional characters. (SOURCE) It seems to indicate that the Malfoys' money comes from inheritance rather than a profession.
In all fairness, the Forbes list is from 2007. Here is the current 2011 Forbes Fictional 15 List, which does not feature our dear Lucius, but can we get a shout-out for SMAUG?!?!
Edited to add 7.13.12 Pottermore came out with a fairly extensive background on the Malfoy family (see end of post if you can't access this link) and according to J.K. Rowling, the Malfoys are indeed independently wealthy and have no need to work.
In spite of their espousal of pure-blood values and their undoubtedly genuine belief in wizards' superiority over Muggles, the Malfoys have never been above ingratiating themselves with the non-magical community when it suits them. The result is that they are one of the richest wizarding families in Britain, and it has been rumoured for many years (though never proven) that over the centuries the family has dabbled successfully in Muggle currency and assets. Over hundreds of years, they have managed to add to their lands in Wiltshire by annexing those of neighbouring Muggles, and the favour they curried with royalty added Muggle treasures and works of art to an ever-expanding collection.
I thought this was interesting, the "suspicion" that the Malfoys dabble in Muggle currency. But here's the part about independent wealth:
It is often said of the Malfoy family that you will never find one at the scene of the crime, though their fingerprints might be all over the guilty wand. Independently wealthy, with no need to work for a living, they have generally preferred the role of power behind the throne, happy for others to do the donkey work and to take the responsibility for failure.
Since not everyone wants to sign up for Pottermore, I've put up screenshots here, here, and here.