Pratt started his amateur career with Hunslet Parkside. At the age of 15, he trained with hometown team Leeds, but was not taken on by the club.[4] He signed for Featherstone Rovers in 1997, and spent two seasons at the club, scoring 30 tries in 48 appearances.[5] Karl Pratt made his début for Featherstone Rovers on Sunday 8 June 1997, and he played his last match for Featherstone Rovers during the 1998 season, he played left wing, i.e. number 5, in Featherstone Rovers's 22–24 defeat by Wakefield Trinity in the 1998 First Division Grand Final at McAlpine Stadium, Huddersfield on 26 September 1998.[6] In November 1998, he returned to Leeds after being signed for a fee worth up to £150,000.[4]
Pratt won caps for England while at Leeds Rhinos 2001 Wales (sub),[2] and won caps for Great Britain while at Leeds Rhinos 2002 Australia, and New Zealand.[3]