He was UNISON's deputy general secretary (DGS) from its formation in July 1993, when it was formed from NALGO, NUPE and CoHSE.

[2] In his role as the deputy general secretary, Prentis directed UNISON's national negotiating team and oversaw the union's policy making functions.

In 2001, he succeeded Rodney Bickerstaffe as General Secretary of UNISON, having been elected in February 2000.

As General Secretary, he received a total salary and benefits package worth £112,114 in the accounting year ending 31 December 2013.

[10] It was announced on 14 October 2022, that as part of the 2022 Special Honours, Prentis would receive a life peerage, sitting for the Labour Party.