Lushington was the second son of Sir Stephen Lushington, 1st Baronet (1744-1807), a Member of Parliament and Chairman of the British East India Company. He was educated at Eton College and Christ Church, Oxford, after which he was elected a fellow of All Souls. He joined the Inner Temple and was called to the bar in 1806. In the same year, he entered Parliament as Whig member for Great Yarmouth, and he spoke in the Commons in favour of the bill to abolish the slave trade in February 1807, and remained a lifelong advocate of the anti-slavery cause. He was re-elected in 1808, but a few months later, after the defeat of a motion he had proposed to castigate the behaviour of Sir Home Popham, he resigned his seat and devoted his energies to his legal practice.
He returned to Parliament as the MP for Ilchester in 1820, and subsequently also represented Tregony, Winchelsea and Tower Hamlets. He continued to support all measures attempting to suppress slavery or the slave trade, and also proposed or attempted to propose motions to recognise the independence of South America from Spain (1820) and to abolish capital punishment (1840). He supported Catholic Emancipation and spoke in favour of repealing the civil disabilities which applied to Jews; he was also a strong supporter of Parliamentary reform, and advocated triennial parliaments and the secret ballot. He retired from Parliament in 1841.
His legal career continued parallel to his political one. In 1820 he was one of the counsel retained by Queen Caroline, and spoke in her defence during her trial before the House of Lords. In 1828 he was appointed judge of the Consistory Court of London. In 1838 he was made a Privy Counsellor and became judge of the High Court of Admiralty, in which post he continued until 1867. He was also Dean of Arches from 1858 to 1867, when he retired from all his posts due to ill health.