Follow Thorn Creek Road to a turnout and parking area at the South Fork of Thorn Creek. Hike a Bike for 1/2 mile through a creek bed with a better and better trail developing along the edges each year. From a major 4 way road junction sometimes referred to as New York Summit, take the jeep road on the prominent southwest ridge to the summit.
Roundtrip it's approximately 25 miles with 5100 feet of elevation gain.
Another possibility involves utilizing the North Fork of Thorn Creek where you'll drive a jeep road toward a prominent saddle that overlooks Idaho City. From this area follow a series of motorcycle trails east and slightly south. Thorn Creek Butte is often visible along the route. Best done as a long trail run. A third option involves using the Middle Fork of Thorn Creek as the approach and is actually the shortest known route to the summit by foot or bike. It's 10 miles roundtrip via the Middle Fork, but this involves using a private road.
Thorn Creek Butte is an excellent cardio training mountain.