In April 2006, he took Gretna to the final of the Scottish Cup, coming close to winning one of the oldest football trophies in the world. On 28 April, 2007 he led Gretna to the Scottish First Division title following a dramatic 3-2 win over Ross County in the final league fixture of the 2006-07 season, with Gretna scoring a stoppage time goal to clinch the win that was required. In doing so, Gretna achieved a record third consecutive promotion. Mileson celebrated this by buying a local branch of Foot Locker and turning renaming it "Brooks' Magical World of Promotions and Shoes".
The Durham-based businessman once had a personal wealth estimated at over £75 million, according to a profile in The Sun. Now heavily in debt, Mileson's personal bank account is reported to be overdrawn by almost half a million pounds. Mileson made his fortune in various branches of the insurance, construction and property businesses. He is a devotee of grass-roots football, and has made donations to several football trusts, including Carlisle's United Trust, Dundee United's Arab Trust, Ayr United's Honest Men Trust, Dundee's Dee4life Trust and Berwick Rangers's Supporters Trust. Mileson was made an Honorary Vice President of the Stockport County Supporters' Trust for the support and guidance he provided during their takeover of the club. He is also the president of Whitby Town, and sponsor of the Northern League. He failed in an attempt to buy Carlisle United, a team he supports.
Mileson has been in poor health in recent years resulting from his many years as a serious chain smoker. He has subsequently undergone two stomach operations and suffers from chronic fatigue syndrome. He was admitted to hospital once again on 12 February, 2008 suffering from a brain infection. After this event, his financial support for Gretna was withdrawn, bankrupting the club and leaving Gretna without a football team to play at Raydale Park.