According to Official Guinness Records,
The greatest distance cycled in 24 hours on a mountain bike is 577.78 km (359.02 miles) and was achieved by Adrian Ellul (Australia) in McLennon Park, Walkerston, Queensland, Australia, from 9 to 10 October 2015. Adrian used a Giant XTC mountain bike for this record-breaking attempt.
For a complete list of 2015 records, please visit 2015 Guinness Records in Australia.