I approached the trailhead for the Petersen Mountains from Red Rock Road which is east of the mountain. Red Rock Road has an exit from Highway 395 north of Reno. Take this exit and go north about 14 miles on this nice paved road to a turn off on the left (west) side of the road that is a gravel road. This isn’t a great road, but I don’t think you need 4WD unless it is wet and muddy. You won’t be able to see the Petersen Mountains to the west until you get to the top of the first hill. Set your trip odometer and go 2.5 miles and turn left. At 3.0 miles take the right fork and at 3.3 miles stay right. At 3.4 miles turn right into the large parking area for the Petersen Mountains.
You may be able to approach this trailhead from Bordertown, but I didn’t use that road and cannot comment on its condition.
This route is not technical from the south and east. Other routes could have places of technical rock climbs. The route is fairly simple once you arrive at the trailhead. Proceed through the gate in the fence and head up the hill. Aim for the green patch of grass where there is a spring. There is enough water to get your feet wet to aim for the left side of the spring. Pick up the trail heading from the spring up to the saddle directly above the spring. The saddle is about a 700 ft climb from the trailhead.
Once above the saddle, pick up the jeep track heading up the mountain. It is easy to follow even though it is overgrown. It hasn’t been used by a jeep in decades. Climb another 700 ft to Horse Spring that is shown on the topo. There is a trough full of water here for livestock or game animals. Continue climbing north another quarter mile and gain another 150 ft in elevation to another saddle. At this saddle you should be able to see the summit a little over a mile north. The route is straight through the valley between this second saddle and the summit. When I was there is was green and grassy with patches of snow still clinging to a few slopes. This is a nice part of the hike that doesn’t gain much elevation.
As you approach the summit area you will see it straight ahead. You have to leave the jeep trail and just climb the rocky slope that gains about 300 ft in a quarter mile. Just pick a route that isn’t too brushy or too rocky. Enjoy the views and return to the trailhead the same way you came up.
Total hike was 6 miles, gained 2,000 ft, and took 3 1/3 hours.
The 10 essentials are always required. There is no fresh water on the trail, so take all you need. A GPS may be handy depending on your navigation skills.