The best trailhead for the hike is the Highline Trailhead located along State Highway 150 33.2 miles east and north of Kamas, or you could drive south from Evanston. You could also start at Mirror Lake, but you must pay to park there, and there isn't as much room to park.
This is the probably the easiest route to the summit of Mount Agassiz. From the trailhead at 10,347 feet, follow the Highline Trail .8 miles to the junction with the Mirror Lake Trail. The trail continues south and east and drops to 9900 feet. Scudder Lake is passed after 2.2 miles (from the trailhead). After another .8 miles, pass a junction with the Wilder Lake Trail. Stay left on the Highline Trail. After another 1.3 miles, turn right at the junction where the Naturalist Basin Trail branches off the Highline Trail. Follow the Naturalist Basin Trail through the beautiful basin and after another mile, turn left at the trail marked for Blue Lake. Follow the trail up to Morat Lakes and then Blue Lake. The peak can be climbed in one long day from this route, but there are several campsites in the area around Morat and Blue Lakes. This is a very popular camping area though. Blue Lake is right at timberline at 10,940 feet and is 6.1 miles from the trailhead. From Blue Lake, head directly north and up the slope to the top of the East Ridge of Agassiz. Once on the ridge, follow it west to the summit. There are some teetering boulders, but not as many as there are on the other routes. The total round trip distance from the trailhead is about 14 miles.
A good pair of boots is needed.