The 66-year-old was convicted of nine charges at the Republic of Ireland's non-jury Special Criminal Court.
The three judges were told how he had significant dealings in cattle and land and received farming grants, but failed to submit tax returns for nine years.
The prosecution was brought after an investigation by the Irish Criminal Assets Bureau (CAB).
Murphy, who lives at Ballybinaby, Hackballscross, County Louth, on a farm that straddles the border with Northern Ireland, had denied the charges.
After a trial lasting 32 days, presiding judge Paul Butler said the court was "satisfied beyond reasonable doubt" that he was guilty.
He was remanded on continuing bail ahead of sentencing in February.
The alleged former IRA "chief of staff", Thomas 'Slab' Murphy, has been found guilty of tax evasion.