David Stone, 11, placed fourth among the 759 runners taking part in the Battersea Park MoRun.
Stone completed the 5km event in 17:26, more than a minute-and-a-half ahead of the next placed runner aged under 20.
Stone’s Shaftesbury Barnet team-mate Lia Radus, 11, placed 19th in the London Youth Games Cross Country Championship at Parliament Hill.