Both are outpaced by Warner Music chief Edgar Bronfman, whose £1,640m bank balance makes him Britain's richest music man.
Cowell's £165m fortune makes him Britain's 11th richest music millionaire – although he is certainly one of the most recognizable in The Sunday Times Rich List 2010.
The X Factor judge signed a new deal with Sony in January that will earn him millions over six years, and his TV shows in the States and the UK fund his transatlantic life and lavish property portfolio.
Less well known is the man at the top of the charts, Edgar Bronfman, 54, chairman and chief executive of Warner Music Group.
A former songwriter who has written for Celine Dion and Dionne Warwick, Mr Bronfman now prefers to stay behind the scenes in the music industry – as does Zomba founder Clive Calder (the UK's second richest music millionaire, with a fortune of £1,300m).
The rest of the top ten in The Sunday Times Rich List 2010, however, are far more recognisable music icons.
The reigning kings of the West End, Andrew Lloyd-Webber and Cameron Mackintosh, take third and forth places respectively.
Lord Lloyd-Webber's new musical Love Never Dies, the sequel to the phenomenally successful Phantom Of The Opera, is due to transfer to Broadway and he maintains his high profile with television talent shows such as I'd Do Anything (hunting for the cast of Oliver) and Over The Rainbow – his current BBC TV show searching for Dorothy and Toto in a West End production of The Wizard Of Oz.
Sir Cameron Mackintosh's £635m fortune makes him the fourth wealthiest music mogul, and 98th overall.
The 63-year-old entrepreneur owns seven London theatres, and plans to endow them with enough money after his death to ensure their survival.
Rock stars also famed for their survival in the music industry make up the rest of the list. Sir Paul McCartney's £475m bank balance makes him the wealthiest musician still currently performing, with Mick Jagger, Elton John, Sting and Keith Richards all in the top ten.
"I feel very young and inspired," said Elton John, 63, when asked about being the seventh most played artist of the decade.
His £185m fortune – up six per cent on last year – has kept him in fine clothes and filled his homes with flowers, but the singer is also a dedicated charity campaigner and philanthropist.
Since 2004, the Giving List has tracked £180m of donations from the Rocket Man singer, and he regularly gives performances in aid of Aids and cancer charities.
Music Millionaires
1) Edgar Bronfman (£1,640m)
2) Clive Calder (£1,300m)
3) Lord Lloyd-Webber (£700m)
4) Sir Cameron Mackintosh (£635m)
5) Sir Paul McCartney (£475m)
6) Simon Fuller (£350m)
7) Sir Mick Jagger (£190m)
8) Sir Elton John (£185m)
9) Sting (£180m)
10) Keith Richards (£175m)
