As you are driving up Sawmill Canyon Road, you'll see a sign on the right saying "Mill Creek Campground." Head up that road a bit until you come up to the trailhead. This trail is restricted to all motor-vehicle access. You can park at the trailhead during the day or pay $5 a night to camp in one of the sites to the East.
This is an out and back trail. The trail starts off easy going and playful. Once you cross the creek, things start to get a little rockier and steeper. There are some really steep lengthy climbs and rocky sections just before Mill Creek Lake.
Once you are to the lake, the trail continues and heads up to some meadows, this is the best part of the trail for mountain bikes. Once you get to the meadow, you cannot find the trail (I spent a lot of time looking). The trail used to go up to Firebox summit and would connect you back to the 101, but has since been overgrown and not maintained due to a fire. The trail is really hard to follow with many downed trees past the meadow.
Coming down is pure bliss! I would only recommend this trail to seasoned riders.