Treslong commanded a ship that became stuck in ice when it was on it's way from Vlie to the island of Texel. His ship was then attacked by 4 companies of soldiers who had been warned by the citizens of the island Wieringen. He managed to defeat them but lost his sword as well as seventeen crewmembers in the battle.
Together with Willem van der Marck he led the the Watergeuzen when they conquered Den Briel on the April 1st 1572. This attack was not their original intention. The watergeuzen were on their way to the Isle of Texel to attack the Spanish fleet they believed to be stationed there. After this they would try to establish one or more strongholds in one or more of the ports in the North-Western area of the Netherlands.
En route they captured several Spanish merchant ships. When they passed the Flemish sands the wind turned and they decided to change their course to Den Briel. At first they merely intended to sack Den Briel, but Van Oranje ordered it to be held against the Spaniards.
In 1573 Treslong became admiral of Holland and in 1576 also admiral of Zeeland. In 1585 he got into an argument with members of the Admiralty of Zeeland about the way in which the beleaguered Antwerpen was to be relieved. As a result he was imprisoned at the city of Middelburg and relieved of his office. Sometime later in 1591 he was released, because of the support of the Count of Leicester.