As a player Richardson's career was divided between Shamrock Rovers and Gillingham. He made his Rovers debut in a 2–1 friendly win over Sunderland at Roker Park on 10 August 1963. His competitive debut came eight days later as Rovers beat Shelbourne 3–0 at Tolka Park in a Dublin City Cup clash. While at Rovers he twice won the FAI Cup in 1968 and 1969 (scoring in the final replay) as well as getting an Inter League cap and represented the club 5 times in the UEFA Cup Winners Cup. He received the first of three international caps for the Irish national team while at Shamrock Rovers, making his debut against Austria in Linz in October 1971. He scored in his final appearance for Rovers on 22 October 1972 at Milltown.
He moved to the English Fourth Division side Gillingham at the age of 25. He played for the Priestfield Stadium side for the next nine years before being released in 1981. During his time there he scored 100 goals.
Richardson's managerial career began in 1989 where he managed Gillingham, having previously served as the club's youth team manager. A rather lacklustre career there ended in 1992, and he returned to Ireland in 1993 to manage Cork City. He proved an instant success bringing the club within striking distance of retaining the league title. However, after 18 months he left the club after a dispute with then chairman Pat O'Donovan.
Following Cork City, he moved on another Cork side Cobh Ramblers before taking up the management position at Shelbourne. His time at Shelbourne saw two Cup successes (1995/96 and 1996/97), one League of Ireland Cup (1995/96) and a runner up position in the League (1997/98) in three seasons. His failure to win the league ultimately cost him his job.
He moved to his old club Shamrock Rovers, where his tenure for three seasons failed to result in a trophy. Ultimately, Richardson and club chairman Joe Colwell disagreed on the direction of the club, and parted ways in April 2002. Colwell wanted to focus on building a new stadium for the club, while Richardson wanted to focus on a full-time panel of players.
In 2005 Richardson returned to League of Ireland management when Cork City's manager Pat Dolan was dramatically sacked before the start of pre-season friendlies. Cork City chairman Brian Lennox signed the 57-year-old Richardson to a two-year contract at Turners Cross 15 days before the first competitive match of the season. Richardson commented that the Cork City job was the only one that could have brought him back to League of Ireland management. Despite being hospitalised in September in his first season in charge Cork City were crowned League of Ireland champions for the second time in the club's history. Cork City also reached the final of the FAI Cup but were defeated by Drogheda United. The 2006 and 2007 seasons saw 4th place positions for his club in both years. In 2007 Richardson led Cork City to FAI Cup victory, but left shortly afterwards following some controversy, and a financial settlement agreement.
Richardson writes a regular blog for the RTÉ / MNS website, and continues to write a weekly column for a Cork newspaper since his time as manager of Cork City.
Richardson has also supported Red FM's national award winning "Break the Cycle" anti-speeding campaign. In addition to his media work, he occasionally teaches football team management seminars. Richardson also once played the bodhran on an album (A Kentish Garland) by Tundra, a folk music group from Kent.