Thomas E. Nevin was born in Bristol, Somerset on 4 October 1906. He was the eldest of seven children born to Thomas Nevin of Cashel, County Tipperary, and Alice Nevin (née Higginson) of Herefordshire. Áine Ní Chnáimhín (1908–2001) who wrote a biography of Pádraic Ó Conaire was Nevin's sister; historian and trade unionist Donal Nevin was his brother.
In January 1936 he married Monica T. M. Morrissey, a UCD graduate in Celtic studies who went on to serve on the Council of the Royal Society of Antiquaries of Ireland and did research on antiquarian matters for Irish History Online. The couple had four daughters together.