From 1605 to 1625 Trumbull was secretary and later envoy from James I and then Charles I at the Brussels Court of Archduke Albert of Austria, ruler of the Habsburg Netherlands.

Early in James I's reign he was a court messenger, and probably he was attached to Edmondes's embassy to the Archduke Albert of Austria, regent of the Netherlands.

On 17 February 1614, he was granted an ordinary clerkship to the privy council; a sinecure, given that Trumbull remained at his post at Brussels.

He died in London in September 1635, being succeeded as clerk to the council by his godson Edward Nicholas, and was buried in Easthampstead church, where a monument was erected to his memory.

While at Brussels Trumbull secured the private correspondence between Francisco de Vargas y Mexia and Cardinal Granvelle on the Council of Trent; an English translation was published in 1697 by Michael Geddes, and one in French by Michel Le Vassor in 1700.