Muhammad al-Mughrabi, a 64-year-old anti-corruption activist, was arrested on 8 August on charges of impersonating a lawyer by practising law without a licence.
Earlier in the year al-Mughrabi was disbarred by the Lebanese bar association in absentia for allegedly bringing the judiciary into disrepute.
But since his release, he told Reuters news service that he had no intention of giving up law.
"I have hundreds of clients that need my help and I will not abandon them," al-Mughrabi said, minutes after leaving jail.
"Absolutely, I will continue my (anti-corruption) campaign. I feel much stronger now than I did three weeks ago," he said, adding that the experience had encouraged him to campaign for better prison conditions.
"I have hundreds of clients that need my help and I will not abandon them"
Rights groups called al-Mughrabi's arrest politically motivated. But Lebanon's public prosecutor has said all his rights were protected.
He still faces six months to three years in jail if convicted.
Al-Mughrabi has worked on politically sensitive cases in Lebanon, such as the defence of property owners of the once war-devastated Beirut downtown against Solidère, the firm entrusted to rebuild the area and the brainchild of powerful Prime Minister Rafiq Hariri.
He has also defended Christian fighters opposing the Syrian military presence and political influence in Lebanon.
Syria keeps some 15,000 troops in Lebanon, where it holds broad political sway.
Human Rights Watch, which had called al-Mughrabi's detention "outrageous" and demanded his release, said the Lebanese bar association had repeatedly tried to disbar him, but that he had continued to register as a lawyer.
Al-Mughrabi's supporters are campaigning to nominate him head of the bar association.