He was admitted to read in the Bodleian Library in 1618, and in 1624 he graduated Doctor of Divinity (DD) at Trinity College, Cambridge, per literas regias.
Hegarty for trial, however, Raghnall Mac Ailein 'ic Iain (Ranald MacDonald of Benbecula), the uncle of the then Chief of Clan MacDonald of Clanranald and great-great-grandfather to Scottish Gaelic national poet Alasdair Mac Mhaighstir Alasdair, intervened and relieved the Bishop and his posse of their captive.
The incident infuriated King Charles I, who sent a furious letter about it to Privy Council of Scotland on 10 December 1630 and was followed by unsuccessful efforts to summon the Highland nobleman to Inverary for criminal prosecution.
The bishop raised a company of foot for the king, distinguished himself as a partisan leader, and conveyed ammunition through from Dublin to Derry.
Leslie is said to have gone to Scotland about midsummer 1642, all the other bishops having previously left Ireland; but he returned after the king's execution, defended Raphoe against the New Model Army just as he had done against the Gaels, and was one of the last Cavalier leaders to submit.
Leslie could get away with this because his wit and aristocratic manners had gained him the covert protection of Henry Cromwell and he was for a time in receipt of a pension.
The slab which covered his remains recorded that the bishop died a centenarian, that he was a doctor of divinity and laws, and that he was a privy councillor to three kings.
In 1638 the bishop was married to Catherine, daughter of Alexander Cunningham, Dean of Raphoe and Marian Murray, who was aged eighteen.