Not sure about mammoths in LaBrea, plenty of other prey animals, along with sabertooth cats which were attracted by what seemed to be easy prey.
ah, here we go... some mammoths and mastodons, but mostly other stuff so far:
http://www.tarpits.org/research/pit91/pitfacts.html
The most common animal fossils that have been excavated from the La Brea Tar Pits are dire wolves (more than 3,000 found) and California's official state fossil, the saber-toothed cat (more than 2,000 found). Other fossils include mammoths and mastodons, ground sloths, camel, short-faced bears, horses, bison and birds. Since Page Museum paleontologists and volunteers began excavating Pit 91 in 1915, more than 250,000 fossils have been recovered. Over 650 different species of animals and plants have been recovered from the tar pits. Of these, most of the fossil plants, insects, snails and small mammals have come from Pit 91.
http://www.tarpits.org/research/mamtooth.html
Mammoth Tooth Replacement
Mammoths, like elephants, grew their teeth in a process much like a conveyor belt. As one tooth was being used and worn, the next was forming and was ready when needed, right behind the one being used. In the case of this fossil from a Columbian mammoth (Mammuthus columbi) discovered in Pit 9, one can easily see the replacement tooth still forming in the jaw behind the existing tooth.
From KFI in L.A.'s reporting this one dwarfs anything from the Tar Pits and, I think, is more complete. There's also word of a possibility this is a NEW species of Mammoth.