Kelvin Jack started his career playing in Trinidad and Tobago with hometown club Trincity United and Joe Public before earning a scholarship to join Yavapai College in Arizona.
Jack was a regular member of the Trinidad and Tobago national football team during the qualifying campaign for the 2006 FIFA World Cup, and travelled to Germany as coach Leo Beenhakker's first-choice goalkeeper.
He put in an inspired performance that day.Jack did go on to play in the Soca Warriors' final group game against Paraguay, which ended in a 2–0 defeat in which Brent Sancho scored an own goal.
Following the 2006 FIFA World Cup, Jack signed for Gillingham and made his debut in a 2–1 home win over Huddersfield Town on 5 August.
[4] In February 2010, Jack joined Southend United on a short-term contract to act as back-up whilst regular reserve goalkeeper Ian Joyce left on trial with an American club.
Shortly after, on 31 August 2011, Jack made a one-off appearance for Isthmian League Premier Division side AFC Hornchurch, playing in a 2–1 loss away to Lowestoft Town after first-team goalkeeper Darren Behcet picked up an injury.
On 11 November 2011, it was announced Jack had again joined up with Maidstone United to cover first-team goalkeeper Charlie Mitten after he suffered a hip injury.
Jack joined the coaching staff of National League side Dover Athletic in the summer of 2020, although he worked behind the scenes until it was announced in early January 2021.
He joined a coaching setup consisting of manager Andy Hessenthaler assisted by Jack's former Gillingham teammate Nicky Southall as well as Liam Ridgewell.
[10] As a member of the squad that competed at the 2006 FIFA World Cup in Germany, Jack was awarded the Chaconia Medal (Gold Class), the second highest state decoration of Trinidad and Tobago.