Reardon was born on 8 October 1932, in the coal mining community of Tredegar in Monmouthshire. Wanting to play snooker, he turned down a place at Grammar School to become a miner at Ty Trist Colliery, aged 14, following in the footsteps of his father. After a rockfall in which he was buried for three hours, he quit mining and became a police officer when his family moved to Stoke-on-Trent, Staffordshire.
Aged 15, Reardon beat fellow Welshman Jack Cowey in the British youth championship. Having won the Welsh amateur title from 1950–1955, Reardon failed to win the English title when he entered the event in 1956 and 1957. Finally upon re-entering the 1964 event, he won the English Amateur title, defeating John Spencer 11–8 in the final. This victory led to an invitation to tour South Africa. This proved to be so successful that Reardon was offered the chance to tour it again as a professional. On the back of this, Reardon resigned as a policeman and duly turned professional in 1967.
Reardon's first appearance at the World Championship was in 1969, and he won his first title the following year, beating John Pulman 37–33 in London. After winning the title, Reardon was in big demand for exhibitions and on the holiday camp circuit. Winning the first ever Pot Black in 1969 made him instantly recognisable; Reardon and John Spencer were the first to capitalise on the snooker boom in the early 1970s.
Reardon also became popular because he added a touch of humour and entertainment to his game. A deadly long potter and tactician in his prime, he played seventeen World Championship matches without defeat and won the title four years in succession. He won his second in 1973 in Manchester when he beat Eddie Charlton 38–32, but the match of the championship was the semi-final between Reardon and Spencer, which Reardon won 23–22.
Reardon beat Graham Miles the following year, and in 1975, when it was held in Melbourne, Australia, he had a tough quarter-final with Spencer which he won 19–17. He then beat Alex Higgins 19–14 before meeting local hero Eddie Charlton in the final. Reardon initially trailed by 29 frames to 23, putting Charlton two frames away from the title, but Reardon then won seven consecutive frames to lead 30–29. Charlton took the sixtieth frame to tie the match but Reardon took the vital sixty-first and won his fourth title.
A special recognition of Reardon's status within the game came in January 1976 when, during the recording of the Ladbroke International Series at Thames Television, Reardon was surprised by Eamonn Andrews, clutching his This is Your Life book. Reardon had actually been suggested as a candidate for the programme by Snooker Scene editor, Clive Everton. The snooker playing fraternity was represented by Alex Higgins, John Pulman, Jackie Rea, Graham Miles, John Spencer, Eddie Charlton, Cliff Thorburn and female professional Joyce Gardner.
Reardon won his fifth title the next year in Manchester, beating Alex Higgins 27–16. Earlier that year, he had won the Benson and Hedges Masters in London, his second snooker title after the World Championship. In this event Reardon defeated John Pulman in the quarter-final 4–1, in a match of such poor quality that the highest break was 22 (by Pulman). Reardon improved his form in the semi-final to defeat Eddie Charlton 5–4 and Graham Miles 7–3 in the final. Reardon would never win the event again but defeated Rex Williams 4–1 and Graham Miles 5–2 in the 1977 event only to lose 7–6 to Doug Mountjoy in the final.
Reardon's unbeaten run at the World Championship ended at the first Crucible championship in 1977, when he lost to John Spencer in the quarter-finals 6–13, his first defeat since Rex Williams in 1972.
Reardon regained the title in 1978 winning it for the sixth and final time by beating Doug Mountjoy 13–9 (after trailing 7–2), Bill Werbeniuk 13–6, Eddie Charlton 18–14 and finally Perrie Mans 25–18 in the final. Reardon remains the oldest winner of the World Championship at the age of 45 years and 6 months. Straight after this Reardon regained the Pontins Professional Title, defeating John Spencer 7–2 in the final.
Towards the end of 1978 Reardon beat Patsy Fagan 6–1 and Alex Higgins 11–9 in a high quality final to win the one-off 'Champion of Champions' event sponsored by the Daily Mirror and held at the Wembley Conference Centre.
1979 saw Reardon regain his Pot Black title by defeating Doug Mountjoy 2–1 in the final. It was Reardon's first win since he won the inaugural event in 1969, although he was runner up in 1970, 1972 and 1980. That same year Reardon joined Mountjoy and then World Champion Terry Griffiths, to win the first 'World Challenge Cup' for Wales, defeating England (Fred Davis, John Spencer and Graham Miles) easily 14–3 in the final.
After Reardon's sixth world title he remained one of the world's best players for some years. He lost to Dennis Taylor in the 1979 quarter-final, and by David Taylor at the same stage in 1980. He went a step further in 1981, beating John Spencer 13–11 and Bill Werbeniuk 13–10 before being surprisingly beaten by fellow Welshman Doug Mountjoy in the semi-finals in what was Mountjoy's best ever World Championship performance. Mountjoy scored a championship record break of 145 during the match, which he won 16–10.
It was the rise of Steve Davis in 1980–81 that saw Reardon and his generation lose their grip on the game. However, despite Davis' domination between late 1980 and the spring of 1982, Reardon remained a major force in the game. At 49, he reached the World final in 1982, losing to Alex Higgins 15–18.
For the 1982/3 season Reardon returned to number one in the world rankings, which in those days were based only on performances at the World Championships over previous years. His career went through an Indian summer, winning the Players Professional Tournament in late 1982, beating Jimmy White 10–5 in the final. He reached the final of the Benson & Hedges Masters, losing 7–9 to Cliff Thorburn, and went on to win the Yamaha Organs International Masters, where he beat Steve Davis 2–1 in the semi-final group stages, before defeating Jimmy White 9–6 in the final, having trailed 5–3. Reardon also regained the Welsh Professional Title, hammering Terry Griffiths 9–4 and Doug Mountjoy 9–1 in the semi final and final respectively. This led him to be fancied in some quarters for that year's World Championship, but ironically he suffered his earliest ever defeat in the competition, losing 13–12 in a high-quality second round match to Tony Knowles.
Reardon also had the distinction of whitewashing Steve Davis twice during the latter's prime. Firstly 6–0 in the 1982 Highland Masters semi-final (an event Reardon went on to win) and secondly in 1988 (see below).
Reardon reached the semi-finals of the World Championships once more, in 1985 (losing 16-5 to Davis). He last played at the Crucible in 1987, losing to Steve Davis again in the second round.
When the snooker world rankings were introduced in 1976, Reardon was the first number one, retaining that position until 1981/1982. His win in the 1982 Professional Players Tournament at the age of 50 led to his recapturing the world number one position in the first set of rankings to be calculated on tournaments other than the World Championship.
Reardon remains the oldest player, at 50, ever to win a ranking tournament. He has also won the State Express World Team Classic for Wales during the first two years of the tournament in 1979 and 1980 with Mountjoy and Terry Griffiths. He also won the Welsh Professional Championship in 1981 and 1983. His last final was the 1985 World Doubles when he partnered Tony Jones and lost to Steve Davis and Tony Meo 5–12. He was awarded the MBE in the same year.
Reardon developed eyesight problems and started wearing 'Dennis Taylor-style' glasses later on in his career. He went out of the top 16 in 1987 but whitewashed Steve Davis 5–0 in the third round of the 1988 British Open. The victory was achieved with Reardon using his old cue with which he had won his world titles, having been encouraged to rebuild it, by Steve Davis. However, in the next round, played under TV lighting, Reardon suffered a drying of contact lenses and lost 5–2 to David Roe, having led 2–1.
Reardon played his last competitive ranking match in the 1991 World Championship qualifying second round. He lost 10–5 to Jason Prince (losing three frames on the black). Upon this loss, Reardon announced that 'there was no bitterness on his part' but he would not be returning. Having slipped to 127th in the provisional ranking and now aged 58, Reardon called a halt to his formal playing career. Reardon's last formal appearance in an event came in the 2000 Senior's event where he lost his opening one-frame match 69–46 to Graham Miles.
Reardon maintains an active interest in the game. He also advised Ronnie O'Sullivan on his way to his 2004 World Championship victory, giving him psychological and technical help.
Reardon now resides in Brixham, Devon. He is currently the president of the golf club in Churston, Devon. Reardon made a playing appearance at a Snooker Legends evening in Plymouth in July 2010.