Like the ship's other junior officers, Boxhall reported to White Star's Liverpool offices at nine o'clock in the morning on 26 March 1912, and travelled to board the ship at Belfast the following day. After the RMS Titanic departed Southampton on 10 April, Boxhall settled into his regular duties. These included scheduled watches, aiding in navigation, and assisting passengers and crew when necessary.
When Titanic collided with an iceberg at 11.40 PM on 14 April, Officer Boxhall was off duty near the Officers' Quarters. Hearing the lookout bell, he headed immediately to the bridge, arriving just after the impact. Capt. Smith, who had also just arrived on the bridge, ordered Boxhall to perform an inspection of the forward part of the ship. He found no damage, but was later intercepted by the ship's carpenter, who informed him that the ship was taking water. A mail clerk confirmed this to Boxhall and Captain Smith. Later, it was Boxhall who calculated the Titanic's position so that a distress signal could be sent out. It was also Boxhall who sighted the masthead lights of a nearby vessel (possibly the SS Californian) and attempted in vain to signal by Morse lamp and distress flares.
Officer Boxhall was put in charge of lifeboat No. 2, which was lowered from the port side at 1.45 AM with 18 persons aboard out of a possible 40. He rowed away from the ship for fear of being pulled down by suction. Boxhall did not actually see the Titanic founder, as her lights had gone off and his lifeboat was about three-quarters of a mile distant. Boxhall spotted the RMS Carpathia on the horizon at 4.00 AM and guided her to the lifeboats with a green flare. After being collected by the Carpathia, Boxhall and the other survivors arrived at Pier 54 in New York on 18 April.
While in New York, he served as a witness in the American inquiry into the sinking. He and his fellow surviving officers were allowed to leave New York on the Adriatic on 2 May. After returning to England, Boxhall bore witness again, this time at the British inquiry. Much of his testimony concerned details of the lifeboat lowerings and Titanic's navigation, including the many ice warnings. He was also the first person to testify that he saw another vessel in proximity while Titanic sank.