His aunt Mary Elizabeth Hallinan married Rupert Watson, 3rd Baron Manton, Senior Steward of the Jockey Club 1982–1985,[2] effectively the chief executive of the British horse racing industry.
Magnier received his formal education at Glenstal Abbey in County Limerick but had to leave school at 15 to take charge of the family estate near Fermoy after his father died.
Magnier began his association with Coolmore in partnership with his father-in-law and champion racehorse trainer, Vincent O'Brien, and Vernon's Pools magnate, Robert Sangster.
His racing empire, which became arguably the most successful on the planet[citation needed]is nowadays powered by a huge string of blue-blooded thoroughbreds trained at Ballydoyle by Aidan O'Brien, plus dozens of others in the care of a number of other trainers.
The strength of thoroughbred talent residing at Coolmore is indicated by the fact that eleven of the fifteen winners of the blue riband of the turf, The Derby, between 1998 and 2012 were sired by Coolmore stallions (High Estate, Fairy King, Grand Lodge, Sadler's Wells (two), Danehill, Montjeu (four) and Galileo)[citation needed] A high-profile venture was his stake of 28.89 per cent shareholding in football club Manchester United F.C., which was sold in May 2005 to Malcolm Glazer, an American businessman.