Standard Route

The usual route up Mexican Mountain is a nice piece of creative writing in the Bjornstad Desert Rock 2 guidebook - probably the only time i've ever seen a route graded IV 5.0.  I have no idea how to rate something like this, but be prepared for a few short cruxes in the 5.6 range with consequences.

From the river crossing, pick your way up slopes to a choss prow.  It can be climbed directly, but a third class short step exists about 100 feet out to the left - if you are scared you are in the wrong spot.  Trudge left and up a huge talus slope to the shortest section of cliff.  Climb this 40' section directly at low 5th class on steep but good rock with big holds. 

Once above the first crux, work your way left and up to the saddle between Mexican Mountain and El Sombrero.  Turn the corner to the south side of the mountain and traverse to another big talus slope, which is steeper and looser than the last one.  At the highest point, climb a clean but unprotectable slab for 50' to a large ledge.  The crux move on this is intimidating and a fall would be very bad.

Traverse right across the ledge to a nice tree.  Head up a steep crack and chimney system for 50' or so.  This can be protected with cams but is steep and physical.  Pick your way up loose slopes and ledges to the huge summit plateau.  

The true summit is a mini butte atop the massive one.  Head right along the base of it to a cairn marking a 4th class corner and slab.  Walk easily to the summit from here, and one of the best views anywhere in the desert!

To descend, reverse the route.  There may or may not be anchors to rap the cruxes, but they can all be rappelled with a single 50 meter rope by slinging trees or boulders.  I'm sure people downclimb the cruxes without a rope, but the consequences of a fall would be catastrophic.

Start after fording the San Rafael River about a mile past the gate closing the Mexican Mountain Road.

A single set of cams from yellow alien up to a 3 camalot is handy. If you want to rappel any or all of the 4 cruxes, a 50 meter rope and a bunch of webbing and a few quicklinks or leaver biners.