From the Trans-Canada Highway, take exit #123 (Prest Road) South to Bailey Road. Turn left onto Bailey rd. In 700m take a slight right onto Elk View Rd. Climb the mountain for 1.7 km and go straight at the 3 way stop. Follow the road about 9 KMs around the beautiful Ryder Lake. Eventually the road will wind around a couple 90 degree turns, then turn into a gravel road. Keep following the gravel road about 1KM and you will see a parking lot on your left. This is the parking lot for the Elk Mountain Trail. And also a popular alternate route to Thurston Mountain. Continue following this road for about 12.5 kms till you see Mt. Thurston Forest Service Road on your left.
At this point, unless you are crazy, Or have a properly built off road rig, and proper wheeling knowledge, Park here.
You can also get to Mt. Mercer via the Elk / Thurston ridge trail, but that would be a very long hike.
The first part of the hike is a steady climb up the forest service road. Be sure to follow the obvious trail leading up at each intersection. There are a lot of logging roads up here. Use your gps and the GPX on this page.
At about 2 kms up the road there will be decent parking lot sized area on your right, that is usually filled with garbage, bullet casings and shotgun shells left by the douche-bags that frequent the area. If you are brave enough to leave your vehicle unattended here, it could shave an hour off your hike. This is the last place you can get before needing a very capable 4×4, If you drive past here, you may be putting your life and the lives of your passengers in danger. If you do park at this spot, please try and help keep BC beautiful and pack out any extra garbage you can.
The hike past this point will show you why an extreme 4X4 would be needed. Between the rock crawling, Dangerous off chamber and extremely slippery and narrow water crossings, and the loose gravel along an extremely steep, You will see why hiking would be the best bet. You will also see the remains of a vehicle that had an inexperienced driver, on route up the mountain. There are decent views of the Chilliwack river valley from this part of the road.
After the road opens up to views south, continue up the forest service road until you come to an open flat spot with views north to Chilliwack. The trail head will be to the right and will look like you can drive part way down. Follow the ridge through the forest and meadow to Mercer Summit. The hike along the actual trail is roughly one hour at a leisurely pace. Great views of the Mt. Laughington and the Cheam Range from Mercer Summit.