Muggleton originally played junior rugby union with Dundas Valley before switching to rugby league at the age of 16 to play for Ryde District Devils, in the Balmain junior competition. Muggleton was initially graded as a fullback with Balmain, but moved to Parramatta in 1980 and the following year, jumped suddenly into first grade just as the Eels won their first premiership. 1982 saw Muggleton become as a second rower an integral part of Parramatta's most dominant NRL team, providing a second kicker to support Peter Sterling, who was later to become his brother-in-law, and showing abundant ball skills to create opportunities for a famous backline of Sterling, Brett Kenny, Mick Cronin, Steve Ella and Eric Grothe, Sr. Muggleton was chosen for the 1982 Kangaroo Tour, which became known as “The Invincibles” as it steamrolled through England and France to win all 22 games, the first time the Aussies had gone through a Kangaroo Tour undefeated. However, after that, Muggleton had a great deal of trouble retaining his place during a succession of injuries and much competition from Mark Laurie, Peter Wynn and Steve Sharp in Parramatta's second row. By 1984, he was used chiefly as a reserve, and in 1985 Muggleton had a spell in the UK playing for Hull FC, alongside Sterling, in the then Rugby League First Division. John Muggleton played left-, i.e. number 11, in Hull FC's 24–28 defeat by Wigan in the 1985Challenge Cup Final during the 1984–85 season at Wembley Stadium, London on Saturday 4 May 1985, in front of a crowd of 99,801, in what is regarded as the most marvellous cup finalin living memory, which Hull narrowly lost after fighting back from 12-28 down at half-time. That year, he played only one match in first grade for Parramatta after returning to Australia, but was recalled successfully in 1986 and, developing as a goalkicker, was able to keep his place and establish himself firmly in 1987 with the retirement of long-standing lock Ray Price. A highlight that year was a booming field goal to beat St. George 21 points to 20 with the last kick of the day. However, in 1988 Muggleton was again out of favour and in his last two years he started only two first grade matches.
Coaching career
After this, Muggleton turned to coaching, being most successful with North Sydney's struggling President's Cup outfit in 1993 where he lifted them from twelfth to sixth. Muggleton was the defence coach with the Melbourne Rebels in the Super Rugby competition, serving from 2011 to 2013. Muggleton left the Rebels at the end of the season. His last game as the defence coach of the Rebels was a home game against New Zealand franchise the Highlanders, a match that also turned out to be the last Rebels match for head coach Damien Hill and players James O'Connor, Gareth Delve, Cooper Vuna, Ged Robinson, Nick Phipps and Nic Henderson. In front of over 12,000 spectators, the Rebels overcame a 24-point half-time deficit to achieve a remarkable 38-37 come-from-behind victory over the Highlanders, ending Muggleton's tenure as a Rebels assistant coach on a winning note. He coached Georgia at the 2011 Rugby World Cup. And, prior to that he was the defensive coach of the Llanelli Scarlets in south-west Wales. On 25 July 2014, Muggleton became the new Defence Coach for English rugby club Gloucester Rugby who compete in the Aviva Premiership Muggleton has been the defence coach at the Parramatta Two Blues Rugby Club since November 2015
