This is the second time within the last few years that I've heard of a Mountain lion roaming Griffith Park. And to make it all the way into the middle of L.A., the local news stated that it would have had to cross the Golden State Freeway. There is a resident wild deer population in the park of somewhere around 200, and plenty of smaller game like rabbits and squirrels.
Griffith Park is huge. To give you a size comparison, New York City's Central Park is 840 acres. Griffith Park dwarfs that at 4,310 acres, for many years the largest municipal park in the US. So you can imagine there is plenty of room for a Mountain lion to hide.