This route is not technically on trail and is not shown on the modern Boulder Open Space & Mountain Park maps. However, an old trail follows roughly this route and it is passable and rewarding for the adventurous. The easiest start is from the NCAR trailhead at the west end of Table Mesa Drive, though other starts are quite feasible.
NOTE: most or all of Skunk Canyon including this route is CLOSED for Raptor Nesting between February 1 and July 31. See the map of closures for more information. Please stay out during these times.
Hike the NCAR trail up past the water tank to the Mesa trail and turn right. After a few tenths of a mile, you'll descend into a gulley and start switchbacking up again with good views of Satan's Slab and the other rocks of Skunk Canyon. At a small bit of fence, leave the established trail and head west into the canyon. Now the fun begins!
There is a rough trail to follow for a little bit. A few hundred feet before the first rock ridge (the Heaven Ridge), the trail becomes extremely hard to follow. Poison ivy is rampant in the summer and there is much slippery and loose rock. Staying on the left (south) side of the streambed may be your best option. Fight your way past the first two ridges (Heaven and Satan's Slab) and scramble up and over a boulder on the right side of the stream. Stay right of the stream from now on.
From here the way is easier. The third and fourth ridges are easily bypassed on a faint trail on the north (right) side and soon you'll break out of the forest into a surpassingly beautiful and lonely grassy cirque. Straight ahead is Green Mountain and the left and right walls are formed by Dinosaur Mountain (a subsidiary peak) and the Skunk Canyon ridges, respectively. Find your way uphill through the grassy meadow trending right as you go and sticking to the higher, less-steep terrain.
Whatever you do, you'll eventually have to scramble up some low-angle scree slopes to attain the ridge at the rocky top of the skunk canyon ridges (directly above the 4th and 5th Flatirons). Head north from here following the ridgeline and admiring the great views. Eventually you'll descend to a small saddle and meet up with the E. M. Greenman Trail. It's an easy half mile from here to the summit.
Descent by this route is not as obvious or safe as the ascent and is not recommended.
No technical gear is required, but sturdy shoes are a must! The main Green Mountain trails can remain icy late in the season. Instep crampons may be a good idea.