From The Mammoth Lakes area, there are three ways to get to Thousand Island Lakes. One is via the High Trail, also known as the Pacific Crest Trail. Another is via the River Trail. This hike description is the route via the John Muir Trail.
From Agnew Meadows, the most direct way to get to the John Muir Trail is to take the same route that is described in the Agnew Meadows to Shadow Lake Hike description. Just past Shadow Lake is the junction with the John Muir Trail.
From the Shadow Creek Trail the JMT heads west for approximately 0.8 miles along Shadow Creek until the trail reaches the Ediza Lake Trail junction. From the junction the trail turns north and heads uphill for the next 2.0 miles and tops out at an elevation 10,100 feet.
From this point, the trail descends about 400 feet along switchbacks until the trail travel alongside Garnet Lake. The trail circumnavigates the eastern shore of Garnet Lake and crosses a bridge over the lake’s outlet until the trail begins climbing again over a series of switchbacks.
The trail again tops out at about 10,140 feet before descending down a series of switchbacks toward Ruby Lake. Along the descent there are some pretty spectacular views of the lake that are somewhat obscured by trees. Soon the trail passes the southeast corner of Ruby Lake and offers a spectacular panorama of the lake.
From Ruby Lake the trail heads uphill for a short distance before descending toward Emerald Lake. Emerald Lake has beautiful and also has open views of the San Joaquin Ridge to the east. Thousand Island Lake is a very short hike over a small ridge from Emerald Lake.
The total mileage from Agnew Meadows to the Junction of the JMT and the High Trail is about 10.6 miles. My track log is 11 miles as it extends to the campsite that I stayed at.