According to the list, Mr Ashley – who founded Shirebrook-based Sports Direct in 1982 – is worth £3.5billion.
Although he remains the richest person in the region, Mr Ashley was the East Midlands’ biggest financial loser over the past 12 months.
While his Newcastle United football business made a £19million profit, Mr Ashley – whose fortune derives primarily from the Sports Direct – saw his fortune drop by £250m overall during the year.
The 50-year-old has a 55 per cent stake in the business which is now valued at £3.7bn, down in value from £4.7bn a year ago.
He once made £929m in a day through the sale of some of his shareholding in the business when it floated in 2007.
Mr Ashley also has stakes in Debenhams, JD Sports, Tesco and MySale and significant property investments.
Sports Direct has faced severe criticism from politicians over its employment practices – with the vast majority of staff at its Shirebrook site coming from Eastern Europe.
Last November, Labour leader Ed Miliband accused the retailer of using “Victorian practices” for hiring thousands of workers on zero-hours contracts.
Speaking at a conference in the West Midlands, Mr Miliband described the company as a “terrible place to work”.
Staff have complained of draconian employment practices, including strip searches and a six-strike dismissal policy.
Sports Direct, one of the UK’s biggest employers, previously said it was continuing to review “core employment procedures”.
Meanwhile, the Duke of Devonshire is the sixth richest man in the East Midlands.
His wealth increased by £50m over the last year to £850m in 2015, according to the list.
The Duke, 70, succeeded to the role following the death of his father in 2004.
He lives with his wife at Chatsworth House – one of Britain’s top tourist attractions.
The Sunday Times Rich List 2015 will be published by the national newspaper at the weekend.
THE EAST MIDLANDS’ TOP 20 RICHEST PEOPLE IN 2015
1. Mike Ashley, £3.5bn
2. Will Adderley and family, £1.280bn
3. Tony Langley, £1.097bn
4. John Bloor, £1.025bn
5. David Ross, £1bn
6. The Duke of Devonshire, £850m
7. Freddie Linnett and the Murphy family, £580m
8. David Wilson and family, £515m
9= Michael Cornish and family, £500m
9= Ashish Thakkar, £500m
11. Sir David Samworth and family, £455m
12. Leon Max, £400m
12= Mel Morris, £400m
14. Tony Wilkinson and family, £350m
15. William Parente and family, £320m
16. John Kirkland and family, £315m
17. Robin Faccenda and family, £314m
18. Michael Gregory, £295m
19. Sir Paul and Lady Smith, £290m
20. Lord Heseltine and family, £280m