Seventeen years old at the time, Jack Thayer boarded the Titanic with his parents, John and Marian Thayer, and a maid at Southampton on April 10, 1912. Jack occupied cabin C-70 while his parents occupied C-68.
Shortly after midnight on 14 April 1912, Jack noticed that he could no longer feel a breeze streaming through his half-open porthole. He did not recall feeling the ship's collision with the iceberg. He dressed and went to A Deck on the port side to see what had happened. Finding nothing, he walked to the bow, where he could faintly make out ice on the Forward Well Deck.
Jack woke his parents, who accompanied him back to the port side of the ship. Noticing that the Titanic was developing a list to port, they returned to their rooms and put on warmer clothes and life vests. They returned to the deck, but Jack lost sight of his parents and after searching for them, assumed they had boarded a lifeboat.
Jack soon encountered Milton Long, a fellow passenger he had met hours before over coffee. Both Milton and Jack tried to board a lifeboat but were denied because they were men. Jack recalled he saw Chief Purser McElroy shoot into the air twice during the shooting at Collapsible C. Jack then proposed to jump off the ship, as he was a good swimmer. However, Milton was not and advised Jack against it.
Eventually, as the ship was sinking quickly, the two men decided to jump and attempt to swim to safety. Milton went first; it was the last time Jack ever saw him. Once in the water, Jack reached an improperly launched and overturned collapsible lifeboat. Too exhausted to save himself, he was pulled from the water. He and a number of other men were able to balance on the boat for some hours. He later recalled that the cries of hundreds of people in the water reminded him of the high-pitched hum of locusts in his native Pennsylvania.
After spending the night on the overturned collapsible, Jack was picked up by Lifeboat 12. He was so distraught and freezing that he did not notice his mother in nearby Lifeboat 4, nor did she notice him. Lifeboat 12 finally made its way to the rescue ship RMS Carpathia at 8:30 am Jack's father did not get to a lifeboat and died.
Nearly all those who lived did so by boarding lifeboats. Jack was one of only about 40 survivors of those who jumped or fell into the water.
Thayer went on to graduate from the University of Pennsylvania. He married Lois Cassatt, daughter of Edward B. Cassatt and Emily L. Phillips. Her grandfather was Alexander Cassatt, President of the Pennsylvania Railroad. The couple had two sons, Edward and John IV, and also daughters. In 1940, he described his experiences with the Titanic's sinking in vivid detail in a self-published pamphlet; 500 copies were printed for family and friends. Oceanographer Robert Ballard used it to determine the location of the Titanic and proved that the ship had split in half as it sank, contrary to popular belief. Thayer's account is sometimes included jointly with the memoirs of the disaster by survivor Archibald Gracie IV in modern editions of Gracie's book Titanic: A Survivor's Story.
During World War II, both of Jack's sons enlisted in the armed services. Edward was killed in 1943 in the Pacific theatre. When the news reached Thayer, he became extremely depressed and committed suicide on September 20, 1945. He was found in an automobile at 48th and Parkside Ave., his throat and wrists cut. He is buried at the Church of the Redeemer Cemetery in Bryn Mawr, Pennsylvania. Thayer was the financial vice president of the University of Pennsylvania at the time of his death. 
Please don't do that. Just give it to somebody else, won't you please. Every one of these ladies has just lost her husband."
—Jack Thayer and Augusta Davies Odgen
Jack Thayer was portrayed by Nicholas Young in S.O.S. Titanic. He first appeared seen reading a book and also observing the passengers in the third class area.
He then tries to find his way to the swimming pool, where he meets Milton Long. Jack then introduced himself to Long, and asked where the pool was. Long, who was peeking at the women in the Turkish Bath, asked Thayer to take a peek as well.
Could you tell us the way to the swimming pool..."
—Jack Thayer and Maude Louise Slocombe
Soon they were caught by Maude Slocombe, scolding them. Thayer tried to defend themself by saying that he and Long were just looking for a way to the pool. Maude then replied curtly and told them to leave immediately.
When the Titanic sank, Thayer and Long debated whether they should jump or slide down into the water. Long chose to slide down, though Thayer warned him he might be sucked down. Long then hesitantly descended into the water, and said goodbye to Thayer.
Thayer survived in the end, while Long did not. On board the Carpathia, he accompanies his mother who mourns the loss of her husband. Mrs. Odgen then came over to them, offering hot coffee and sandwiches. Thayer told Mrs. Odgen not to do it and to give it to someone else, because both his mother and many of the women next to him have just lost their husbands. Mrs. Odgen replied to Thayer that he could understand their feelings, and tried to comfort Mrs. Thayer and Mrs. Astor.
Charles McAteer played Jack Thayer in the 1997 production of Broawday; in the musical he is seen as a child of about ten when in reality he was seventeen at the time of the shipwreck.
After some time, his mother literally comes to get him. She takes Jack away, leaving Georgiana without a partner for a moment.
When the ship is sinking, Jack is denied access to a lifeboat by Fifth Officer Lowe. Marian seems to be concerned about him, as he's "just 17", but the boat is lowered without him.
Later on during the sinking, he meets up with Milton Long and decides to make a swim for it. Milton wants to slide down the side of the ship, but Jack wants to jump clear from the Titanic. His decision to jump away from the Titanic saves his life.
Following the sinking of the Titanic, he swims to Collapsible B where Seaman Davis refuses to allow him on the lifeboat saying there is no more room. After a man Mario thought was his brother dies, he lowers his body off the upturned boat and allows Jack onto the boat.
- Philip Sherwell (March 26, 2012). "Down and down I went, spinning'". Vancouver Sun. http://www.vancouversun.com/story_print.html?id=6358791&sponsor=escapes.ca.
- Mr John Borland Jr. Thayer - Titanic Biography - Encyclopedia Titanica
- Marshall, Logan. "Sinking of the Titanic and Great Sea Disasters". Project Gutenburg. http://www.gutenberg.org/ebooks/781. Retrieved April 22, 2009.
- "JOHN B. THAYER 3D FOUND DEAD IN CAR; Philadelphia Leader's Throat and Wrists Cut—Had Grieved Over Son's' Death in War". The New York Times. September 22, 1945. http://select.nytimes.com/gst/abstract.html?res=F60C1FF6345F1B7B93C0AB1782D85F418485F9.