By occupation he was a school teacher in Hull, but Jack was best known as being a masterful rugby player. He played four seasons with Hull FC (also known as the "black and whites") where he still holds the record for the most tries (akin to touchdowns in American football), 52 in the 1913/1914 season.
In 1915, Harrison joined the military and trained as an officer. The following year, he was commissioned as a 2nd Lieutenant of the 6th Platoon, 11th Battalion. In late March of 1917, he was awarded the Military Cross (Britain's third highest award) for leading his platoon through No Man's Land, reaching his objective and capturing a prisoner.
Just a little over a month later on May 3, 1917, at the Battle of Oppy Wood, near Arras in Northern France, Harrison again demonstrated extreme gallantry under fire when his platoon, which had been twice repulsed, was pinned down a third time between the trenches by heavy machine gun fire. Realizing that everyone would die unless action was taken, Harrison singlehandedly charged across an open field and reaching the machine gun emplacement, silenced it with a grenade. Sadly, he was never seen again, nor was his body recovered after the battle. For this action, his widow Lilian was presented his Victoria Cross (Britain's highest honor) by King George V at Buckingham Palace.
Jack's only son, Jack, Jr., was later killed in the defense of Dunkirk in World War II. In late 2003, a memorial was erected outside the stadium where Hull FC plays honoring Jack Harrison.