If you TOOK MY ADVICE and parked along Mapleton, walk west on the sidewalk past a few parking spots on the street. You'll come to this break in a wooden fence. Your hike begins here!
You'll know you're in the right place by this sign. It is a bit misleading; it only mentions the Sanitas Valley Trail, but this is a general access point for any of the Sanitas trails you should choose. Boulder's Open Space and Mountain Parks (OSMP) team is extremely diligent about their signage. This sign communicates that dogs are allowed on this trail segment on-leash, bikes are not allowed, and horses are allowed (although I've never seen one here).
Walk though a short wooded section along a ditch. This is perhaps the narrowest part of your entire route. Accordingly, watch out for trail runners and people talking on phones, as they rarely yield, especially when they are near the end of their journey.
In a few hundred feet you will come to a large trail junction. Take the trail to the left of this sign. There is a ton of stuff here, and it totally makes sense for you to be overwhelmed by it. You will find: a map of the area's trails, color-coded by dog leash regulations; announcements and other regulations; a sign displaying the dog regulations in the immediate area; temporary signage; free, compostable dog-poop bags; a garbage can for ONLY dog poop in said compostable bags; and a trash can for all other trash, including dog poop in non-compostable bags. Overwhelming? Yes. Helpful? Also yes.
Stay right at this junction to continue uphill on some steps.
This is the last true trail junction before you reach the summit. The rest of this route description details the features of the trail, and how not to end up off-trail...but you would not get lost if you stopped reading right now. Before you go, though: looking to get down from the top? You have options.
Going back the way you came on the Mount Sanitas Trail is always enjoyable, and it is the fastest.
On the other side of the summit, you have the East Ridge route. That way is a steep, scramble-y descent that quickly drops you into the valley, where you will have your choice of gentle trails back to the parking lot. Take the Dakota Ridge Trail for a shady, rolling ridge run; or the Sanitas Valley Trail for a straightforward, wide, SUNNY gravel trail that trucks can (and do) drive on.
If you are a trail runner, or if you feel like a little more solitude, take the Lion's Lair route, a dog-forbidden trail that gently loses the elevation as it switchbacks into Sunshine Canyon.
Now, for those still curious: the remaining description of the Mount Sanitas Trail!
Cross a bridge over Silver Lake Ditch (great for doggie dips), and well-behaved dogs who are registered in Boulder's Voice and Sight Program may go off-leash.
The trail is a little mean for a minute here. It is just steep though, not bad trail. All the trails on Mount Sanitas are pretty well-constructed, and for good reason! They receive a ton of foot traffic, and those millions of footsteps can do massive damage to a poorly built trail. Thank you OSMP!
You will see a lot of trails branching off from the obvious main trail. But in general, off-trail use is discouraged except where designated by signs like this. Sanitas is loved by hikers and climbers alike, and climbing access trails are well-marked.
An example of OSMP's awesome trail management: these workers were just finishing up a new stone staircase! The trail through here previously did not have any step; it was just sloping, sandy soil. It was not a huge deal on the way up, but coming down it was very easy to lose one's footing. This type of trail improvement is just perfect if you ask me: it makes the trail's route very clear, and it makes everyone's hike easier and safer.
This is not the first available view of Boulder you will come across, but it is pretty representative of what you will all along Sanitas' south ridge. Dakota Ridge is in between you and the city, with Sanitas Valley between Dakota Ridge and you. You could yell down to people walking the Sanitas Valley and Dakota Ridge trails if you felt so inclined.
This photo is here simply to remind you that it isn't all steep stairs. Sanitas has a way of doling out the pain with some intense uphill, with flat sections to catch your breath. Enjoy.
If you are with your pooch, you might find them instinctively checking out certain spots with gusto. My best guess is that lots of dogs pee in these spots. They get to sniff who has come through, and they get to leave their stamp, too. Dog social media. I call these spots shrines. This is Murpy checking in at her favorite one.
The trail stays pretty moderate over some sloping earth and stone steps. When you see this abrupt right turn, the false summits are over and you are almost there.
At this point the Mount Sanitas Trail joins the Lion's Lair Trail (coming from the left) for a few feet to the summit.
You made it! There is a metal pole and a rock with a plaque to commemorate the summit. On the map, the endpoint is a bit off - that marker represents the Mount Sanitas Trail-Lion's Lair Trail junction. But walk like 50 feet uphill and you are there. Enjoy the views, and chat with the occasional OSMP ranger as well as some of Boulder's fittest people. Plenty of ultra-runners regularly do laps on Sanitas.