Born in Leeds, Peacock is a product of Bradford's junior programme, having played as a junior for Stanningley RLFC. On his way to becoming a first team regular for Bradford, Jamie spent a month on loan at Featherstone Rovers making four appearances and scoring one try. He also enjoyed a spell at University of Wollongong in Australia as he learned his rugby trade.
He made his Super League début in 1999 for Bradford, making two starts and 16 substitute appearances for the club, scoring six tries. However, it was the following year that he really cemented his place in the side, helping Bradford to Rugby League Challenge Cup success over Leeds at Murrayfield. During his time with Bradford he won every honour available, with Grand Final, Challenge Cup and World Club Challenge honours to his credit with Peacock recognised as being one of the key ingredients of this success. He gained a reputation as one of the best forwards in world rugby[dubious– discuss] and is a player who has gained respect[dubious– discuss] from not just the fans but the critics as well as his peers, winning such accolades as Man of Steel, Players’ Player of the Year and Rugby League Writers’ Player of the Year.
Peacock earned himself a place in John Kear’s England side for the 2000 World Cup. He made four appearances in the World Cup that year, scoring an impressive six tries which including a hat trick against Fiji at Headingley. Peacock played for the Bradford Bulls at second-row forward in their 2001 Super League Grand Final victory against the Wigan Warriors. Peacock played for England in 2001 against Wales. Also In 2001, he capped the season by making his Great Britain début against Australia scoring a try after just 86 seconds of his début in the first test victory at Huddersfield. He then become a permanent presence in the Great Britain squad making 14 appearances, all but three of which have been in the starting line up. Peacock won caps for Great Britain while at Bradford in 2001 against Australia (2 matches), and Australia (sub), in 2002 against Australia, New Zealand, and New Zealand (sub) (2 matches), in 2003 against Australia (3 matches), in 2004 against Australia (3 matches), and New Zealand, in 2005 against Australia (2 matches), and New Zealand (2 matches).
As Super League VI champions, the Bulls played against 2001 NRL Premiers, the Newcastle Knights in the 2002 World Club Challenge. Peacock played at second-row forward in Bradford's victory. He made 30 appearances as Bradford again returned to Old Trafford for the 2002 Super League Grand Final only to lose out to a Sean Long drop goal. However, Bradford were unstoppable as they captured the Challenge Cup, League Leaders and Grand Final trophies ahead of Leeds and Wigan respectively and Peacock was named Players Player of the Year, Rugby League Writers Player of the Year and Man of Steel.
Peacock was made captain of Great Britain for the 2005 and 2006 Gillette Tri-Nations tournaments due to injuries to Paul Sculthorpe. Leeds born Peacock joined the club he supported as a boy in 2006 from local rivals Bradford. He made his Leeds début against Huddersfield on the opening day of the season.
He played for England in 2008 against France. On 16 June 2008, Peacock was announced as the England captain for the 2008 Rugby League World Cup. He was named in the England squad for the 2008 Rugby League World Cup. He was selected for the England squad to compete in the 2008 Rugby League World Cup tournament in Australia. Group A's first match against Papua New Guinea he played at prop forward in England's victory. He also played against Papua New Guinea, Australia, and New Zealand (2 matches), in 2009 against France (2 matches), Australia, New Zealand and Australia. Peacock played a starring role in Leeds' 33-6 Grand Final defeat of then reigning champions' St Helens. He played a full 80 minutes to win his 4th Super League champions ring. He has been named in the Super League Dream Team for both the 2008's Super League XIII & 2009's Super League XIV season. He was selected to play for England against France in the one-off test in 2010.
Jamie signed a new one-year deal at Leeds in July 2011. Later that year he played at prop forward for Leeds in the 2011 Challenge Cup final which was lost to Wigan.