After visiting the chief medical schools on the continent, he returned to Ireland in 1788; but the sudden death of his elder brother, Christopher Temple Emmet (1761–1788), a student of great distinction,[2] induced him to follow the advice of Sir James Mackintosh to forsake medicine for the law as a profession.

[3] Emmet was a man of liberal political sympathies and became involved with the campaign to extend the democratic franchise for the Irish Parliament and to end discrimination against Catholics.

[3] When the Dublin Corporation issued a declaration of support of the Protestant Ascendancy in 1792, the response of the United Irishmen was their non-sectarian manifesto which was largely drawn up by Emmet.

Though not among those taken at the house of Oliver Bond on 12 March 1798 (see Lord Edward Fitzgerald), he was arrested about the same time, and was one of the leaders imprisoned initially at Kilmainham Jail and later in Scotland at Fort George until 1802.

Upon his release, he went to Brussels where he was visited by his brother Robert Emmet in October 1802 and was informed of the preparations for a fresh rising in Ireland in conjunction with French aid.

He received news of the failure of Robert Emmet's rising in July 1803 in Paris, where because of his alleged Jacobin sympathies and complaints regarding broken French promises he had lost out to Arthur O'Connor as the favoured Irish emissary.

[1] On his arrival, he wrote to Robert Simms in Belfast: "France is the headquarters of fraud, deceit, despotism and under its present rulers no Nation or People who love liberty need look for its honest cooperation".

His abilities and successes became so acclaimed and his services so requested that he became one of the most respected attorneys in the nation, with United States Supreme Court Justice Joseph Story declaring him to be "the favourite counsellor of New York.

[16] Emmet died on 14 November 1827 while conducting a case in court regarding the estate of Robert Richard Randall, the founder of Sailors' Snug Harbor, a home for needy seamen in Staten Island, New York.

His direct descendant, French human rights lawyer Valentin Ribet, died aged 26 in the terrorist attack on the Bataclan in Paris on Friday 13 November 2015.

His grandson, Dr. Thomas Addis Emmet, a prominent doctor and Irish American activist, requested that he be re-buried in Ireland so he could "rest in the land from which my family came."