Humphreys started playing rugby while at Ballymena Academy,[1]: p. 17  and represented Ireland Schools, captaining them to the Triple Crown in 1990.

[9][10] After rugby union was declared open to professionalism in 1995, Humphreys was approached by Clive Woodward to sign for London Irish,[1]: pp.

[17] Humphreys continued to be selected for Ulster while at London Irish, starring in a win against New South Wales in February 1996, in which he scored 17 points - a try, three conversions, a drop goal and a penalty.

[21] After Mark McCall sustained a serious neck injury, Humphreys was named Ulster's captain, and led them to win the 1998–99 Heineken Cup.

[22] He won the All-Ireland League with Dungannon in 2001, and was man of the match in the final, scoring five penalties, four conversions and a drop goal, 26 points in all, against Cork Constitution.

[24] He set a Heineken Cup record for the most points scored in a single game, with 37 in Ulster's 42–16 win against Wasps in 2002.

[35] He also represented Ireland at the 1997 Rugby World Cup Sevens,[39] and played six times for the Barbarians between 2003 and 2005, once as captain, and scored 33 points.

[41] His role included player recruitment, and he is credited with signing South African stars Ruan Pienaar, Johann Muller and John Afoa,[42] and persuading Tommy Bowe and Roger Wilson to return to Ulster.

He received an Honorary Doctorate from the University of Ulster in December 2003 for Services to Sport[54] and an MBE in the Queens New Year's Honours List in January 2004.