Beak joined the Royal Naval Volunteer Reserve as a rating on 2 February 1915, but before seeing action was commissioned as a temporary sub-lieutenant in the Royal Naval Division in May 1915. He was posted to the Mediterranean Expeditionary Force, but it is not clear from his service record if he saw action in that theatre. He arrived in France in May 1916, and after being appointed adjutant of the Drake Battalion on 2 March 1917, he ended up commanding his battalion as an acting commander between 19 March 1917 and 3 April 1917. He was promoted to temporary lieutenant commander on relinquishing command, and attached to headquarters.