Getting There: The best route to get to the trailhead is to take the Little Joe Road (#282) out of St. Regis and then take the state line road (#391) at the Idaho / Montana summit. Follow this road for approximately 5.5 miles. The trailhead is about a mile past the Square Lake trailhead. The trailhead for trail #250 is on the shoulder of Ward Peak and has signage. There is a wide spot just back from that point or you can park at another pullout just down the road. 
There is another road from the Idaho side that connects just down from there to turn around.
The state line road (#391) gets considerably narrower and rougher at that point. If you are tempted to follow the road out to eventually get to the Hiawatha Trail, make sure you have a couple hours to spare, a servable spare tire and a chainsaw. The road is very rocky and low-slung cars are not recommended. Razers, jeeps and side-by-sides use the road and do not cut out the deadfall to standard truck width so you may have to cut out an opening to get your truck through. I have seen cars on the road, but I wouldn’t take one.
Take the Ward Peak trail (#250) and stay to the right (upper) at the trail split. The lower trail (#250) is the Up Up Mountain ridge trail that will take you over Eagle and Gold peaks and eventually to the road to Up Up Lookout.
The upper trail is not numbered and will take you to the summit of Ward Peak.
At the point that this upper trail connects with the ridgeline and turns back to the Ward Peak summit; leave the trail and head down the ridge toward the low saddle between Ward and Eagle peaks. There is a primitive trail of sorts along the ridge, but I would consider this a cross-country venture the rest of the way to Mary Lake. You’ll be able to see the lake the entire way.
When you get to the dividing ridge between Mary Lake and Hub/Hazel lakes dive off there and find the best route you can take down to Mary Lake. There is no trail, and it is steep. It is tempting to try and stay on the rocky ridge, but it seemed that there would be some places that would be more difficult than the green draws that lead to the lake.
At the Lake: Mary Lake has no fish, and the only established campsites that I have seen is over by the very buggy outlet. I really liked this lake as you get some unique views of both Eagle and Ward Peaks, but you also get some outstanding views looking directly down into Hub Lake and Hazel Lake out in the distance. It is well worth the effort to visit Mary Lake.
Follow the same route out that you took to get to Mary Lake but do take the time to go to the top of Ward Peak. The peak is one of the tallest for this area of the state line and has some very good views. There are remnants of the lookout tower (cabin) that sat there years ago.
Bonus Route: There is a primitive trail between Mary Lake and Trail #280 that climbs up from Hub Lake to the saddle and Trail #250 between Ward and Eagle Peaks. If starting at Mary Lake the trail can be found thus: Head directly up the ridge overlooking Hub Lake, angle a bit to the right as it gets steep and the trail starts there and is well defined but un-marked. If finding the primitive trail from the Trail #280 side: There is a nice small flat with 2 very large downed trees towards Mary Lake, the trail is found to the left of the trees (See Pics). It is not very clear until you get around the trees. Trail #280 makes 2 short switchbacks before resuming a unrelenting steep climb to the saddle, about 300 feet of gain.
The trail not too difficult to navigate as it is fairly visible most of the way. There are several short sections of swampy / boggy areas along the way. Don't overthink these as the trail picks up again directly across the areas, just go straight across the wet areas. There is a good spring about halfway across the traverse that has enough volume to filter. From the trail #280 side, the trail will mostly climb up to Mary Lake but it's not that steep. Just take your time and be patient if not familiar with the trail.
