He was admitted an advocate in
Doctors' Commons in London on 28 April 1668 and began practising in the ecclesiastical and admiralty courts. He would remain for the next 15 years, becoming a lawyer of high repute. Through the offices of his father-in-law, Sir
Charles Cotterell, he was appointed chancellor of the diocese of Rochester in 1671 by its bishop,
John Dolben, the future archbishop of York, and benefited much from 'the friendship and patronage of that great and good man'. In 1683 he was appointed
Judge Advocate of the Fleet by Lord Dartmouth,
George Legge, in an expedition to evacuate the British colony at
Tangier, where he was to act as commissioner for settling the leases of the houses between the King and the inhabitants.
Samuel Pepys, who was also on the expedition, was unimpressed – "Strange to see how surprised and troubled Dr. Trumbull shows himself at this new work put on him of a judge-advocate; how he cons over the law-martial and what weak questions he asks me about it." Later Pepys calls him "a man of the meanest mind as to courage that ever was born." Dryden records, in the postscript to his translation of
Virgil, that "if the last Aeneid shine amongst its fellows, it is owing to the commands of Sir William Trumbull, who recommended it as his favourite to my care." It was Trumbull who, admiring Pope's translation of the "Epistle of Sarpedon" from the
Iliad urged him to translate the whole of
Homer's works, and Pope's "Spring" was dedicated to him. ==Family life==