Born in Finglas, County Dublin, Twomey played both hurling and Gaelic football with his local club, Erins Isle.
His father, Tom, was from the hurling stronghold of Newtownshandrum in County Cork and served as chairman of Erins Isle, while his mother, Maureen, was club secretary for a number of years.
John currently lives in Monkstown, County Dublin with his wife, Kathleen, and their three sons, Dara, Jack and Luke.
In 1983, when John was just 21 years of age, he helped Isles win their first and only Dublin Senior Hurling Championship, beating Ballyboden St Endas in the final.
In 1990, to celebrate the centenary of their All-Ireland victory, his grandfather's club, Aghabullogue, hosted a hurling match against Castlebridge, in which John took part.
[7] The current panel had been under the guidance of Boggan for many years now and were very experienced, including the likes of Brian McMahon, Vinny Holden, MJ Ryan, Shay Boland and Peadar Carton Snr.
In the final, they played Offaly at Croke Park but were beaten by 1-19 to 2-11, Twomey managing to contribute a point from the corner-back position.
[7] Twomey won a Railway Cup medal as a substitute with Leinster in 1988, along with fellow Dublin teammates Brian McMahon, Shane Dalton and John Treacy.