He was educated at Trinity College Dublin, entered the Middle Temple, and was called to the Irish bar in 1830.

Whiteside very rapidly acquired a large practice, and after taking silk in 1842 he gained a reputation for forensic oratory surpassing that of all his contemporaries, and rivalling that of his most famous predecessors of the 18th century.

He defended Daniel O'Connell in the state trial of 1843, and William Smith O'Brien in 1848; and his greatest triumph was in the Yelverton case in 1861.

[citation needed] In the same year he was appointed Lord Chief Justice of the Queen's Bench,[1] having previously turned down offers of a junior judgeship.

Barristers who practised before him said that his charm, courtesy and constant flow of jokes made appearing in his Court a delightful experience.