Occasionally, Hal would hunt for food in the North Moors of the Shire. One day, Hal saw what he called a "tree-man" in the North Moors. The tree-man was revealed to be a Huorn who was hunting for food. At the age of eighteen, Halfast fought in the Battle of Bywater against the Ruffians. Considered to be very tall and skilled, Halfast took down a number of the Ruffians and proved himself be strong. During Sam's discussion with Ted Sandyman, Halfast mentioned this happening during Bilbo's 111th birthday party on September 12, 3001 3A. After his cousin left the Shire, Halfast and several members of the Gamgee family grew worried about his departure.
During the War of the Ring, Halfast received word from Will Whitfoot that Sam, Frodo, Pippin, and Merry were in Rivendell and travelled with the Fellowship of the Ring to Mordor and destroy the One Ring. Halfast went on to join the Shire Company and travelled south to where he will face the upcoming conflict. As he arrived to Gondor with the company, Halfast participated in the Battle of Pelennor Fields and slew many orcs that were retreating from the battlefield. After the battle, Halfast volunteered in gathering the wounded and taking them to Minas Tirith to be healed. He was very happy to see Merry and Pippin, but was worried when he learned that his cousin was in Mordor. Later, Halfast traveled to the Black Gate with the armies of the Free Alliance and prepared for their final offense against Sauron's forces. He was among those that were killed during the Battle of Morannon.