Martin first joined the Canaries on 26th February 1981 from Nottingham Forest as Ken Brown invested money in a vain attempt to stave off relegation from the old Division One. Martin had been with Forest since 1971 when he came over from his homeland of Northern Ireland where he had studied law and played for Distillery and Derry City. A total of 371 appearances for Forest resulted in 62goals and international honours with Northern Ireland. Eventually he would gain 64 international caps. He sought a way out of Forest when they requested he played on the left when he preferred a creative central midfield role.
In addition to the £250,000 that Ken Brown splashed out on O'Neill, Brown also paid £175,000 for defender Steve Walford and brought goalkeeper Chris Woods on loan from QPR.

Despite their best efforts, Norwich were relegated on the last day of the season in a 3-2 defeat at home to Leicester. O'Neill's contract had an escape clause which stated he could leave if Norwich were relegated and on 25th June 1981, he joined Manchester City for 275,000. His spell at Maine Road was unsuccessful and Martin made just 3 appearances before returning to Carrow Road in February 1982 for£125,000. Adding to his initial 11 league appearances for the Canaries, Martin played a valuable role in Norwich's late surge up the Second Division table and into the 3rd promotion slot behind Luton Town and Watford. In his 20 appearances, Martin struck 6 goals from midfield.

The following summer saw the highlight of his international career when he captained Northern Ireland at the World Cup finals in Spain. The Irish side being one of the surprises of the tournament and topped their first round group after beating Spain 1-0.

Martin made 35 appearances for Norwich in Division One in season 1982/83 scoring 5 times. He also made 3 FA Cup and 5 league Cup appearances scoring in one of the latter. In August 1983, Martin moved back to Nottingham to join Notts County for whom he made 64 league appearances scoring 5 goals. A brief stop at Chesterfield in 1984 preceded an even shorter visit to Fulham where finally he retired from a knee injury in February 1985.

In August 1987, the lure of football was too strong and he returned to the game as manager of non-league Grantham Town. In July 1989, he moved to Shepshed Charterhouse as their manager before taking on the hot seat at Wycombe Wanderers the following February. In the five and a half years that he spent at Wycombe, Martin gained respect for his managerial attributes as he brought Wycombe the FA Trophy twice (1991, 1992), the Bob Lord Challenge Trophy (1992) and finished second in the GM Conference in 1992. The following year saw Wycombe promoted to the Football League by winning the Conference with a massive gap of 15 points between them and second place. Although linked with the vacant manager's position at Notts Forest after Brian Clough departed that summer, Martin was content to see how his Wycombe side would fare in the Football League. The result was promotion to Division Two via the Play-off finals at Wembley. Wycombe narrowly missed promotion the following season.

In the summer of 1995, Martin decided it was time for a change and was tempted to the vacancy at Carrow Road with the target of a quick return to the Premiership following the relegation the previous May. He brought with him Assistant Manager Paul Franklin and Reserve Team Manager Steve Walford. However things did not work out as Martin would have liked. Although the Canaries were constantly in the top six of the table, struggles with chairman Robert Chase over funding for team strengthening eventually came to a head. On Sunday 17th December, fans traveling to Filbert Street for the clash with managerless Leicester were shocked to hear that O'Neill had resigned. To make things worse, Norwich threw away a 2 goal lead to tumble 3-2 to one of their promotion rivals. The following week saw O'Neill confirmed as Leicester's new boss (He replaced Mark McGee who had joined Wolves). Franklin and Walford eventually joined him in the East Midlands.
Whilst Norwich struggled and even flirted with relegation under Greg Medeson leadership, Leicester went from strength to strength. Although unpopular with the Filbert Street faithful at first, Martin led Leicester into the promotion play-offs and victory over Crystal Palace saw them promoted to the Premiership.

Martin worked more miracles in his first full season in charge as relegation favourites Leicester won the Coca Cola Cup and finished a respectable 9th place. This season (1997/98) has seen Leicester enjoy UEFA Cup matches and again occupy a placing in the top half of the table with victories at Old Trafford and against Chelsea. Although one of the worse-paid managers in the Premiership, Martin is respected for his tactical nous and ability to conjure results out of limited resources.
Leicester's mid-table finish in 1997/98 and boardroom upheaval at Leicester suggest that Martin O'Neill may find it the right time to move onto a bigger club. So far this summer, he has been linked with Liverpool, Everton and Celtic. Martin has also been commentating on the World Cup for BBC TV.

In early September, Martin found himself again linked with a move to another club as he was seen as a viable replacement for Christian Gross who had been sacked by Tottenham. However the media's speculation overdrive had also linked Glenn Hoddle, Joe Kinnear, George Graham and Raddy Antic to the Spurs' job. Eventually George Graham accepted the post leaving Leeds United to take over at White Hart Lane. Martin has been linked with the vacancy at Elland Road and has suggested in media interviews that he would be interested in this job. Whilst the media expects O'Neill to join Leeds, the tabloids have started speculating on his potential transfer targets with the Sunday People suggesting Neil Lennon (Leicester) and former Canary Ashley Ward . Norwich's transfer listed Keith O'Neill is another possibility. All of this has speculation proved to be in vain as Martin decided on Wednesday 21st October to reject Leeds offer and instead stay at Filbert Street. The pleas and demonstration by the Leicester fans at the game with Tottenham on Monday 19th October obviously helped sway his opinion. It is pleasant to see some loyalty within the footballing industry.
Martin led Leicester to the Worthington's Cup final but lost 1-0 to Spurs. Despite rumours of his Leicester side splitting up this summer due to expired contracts and the attentions of bigger clubs, Martin has accepted the challenge of Filbert Street and has signed a two year extension to his contract. According to the Daily Mirror (17th May 1999) Nottingham Forest's new chairman wants O'Neill and assistant John Robertson to take over at the City Ground. This proposal is popular with the Forest fans who chanted that "O'Neill is a Forest Fan" on their last game of the season against Leicester. Rumours that O'Neill is unsettled are not helped by the fact that he is yet to sign his new contract whilst there is a long-running feud between him and Leicester's Chief Executive Barrie Pierpoint.

On 10th December 1999, Martin returned to Carrow Road to sign Darren Eadie for Leicester's record transfer fee of £3million. On 3rd January 2000, Martin ruled himself out of the race for the vacant Northern Ireland manager's position stating that it was too big a task to do part-time and that running Leicester City took all of his resources and intellect and that there was no time spare for researching international opposition.

In February 2000, John Barnes' was sacked as Celtic manager following their shock defeat at home to Inverness Caledonian Thistle in the Scottish Cup. O'Neill was linked the following Sunday to the vacant position at Celtic with The People claiming that he "has jumped to the top of Celtic's manager hitlist but that it will cost them £1million to drag him away from Leicester. And Celtic will have to wait until the summer before launching their bid.. there is a clause [in O'Neill's contract] which would allow him to switch to a major club for a compensation fee just short of seven figures. And the claus can only be activated in the close season". The People claim that O'Neill would be receptive to such an approach because he would love to manage a bigger club, he has become disillusioned with the boardroom struggles at Filbert Street and that star players Emile Heskey and Muzzy Izzet will probably depart this summer. This rumour was also supported by the Sunday Mirror on February 13th 2000.

Martin's Leciester City side won the Worthington's Cup final at Wembley on 27th February 2000, defeating John Aldridge's Tranmere 2-1. This gives Leicester a place in the 2000/2001 UEFA Cup campaign. At the end of May 2000, Martin was closely linked with the vacancy at Celtic and was interested in the post. On 1st June 2000, Martin signed a three year deal worth £1million a year as Celtic's new manager despite attempts by Leicester chairman John Elsom to keep Martin at Filbert Street. Elsom felt that Martin "was making a wrong move and that his career would be better served at Leicester... But I must say Leicester have had a marvelous sequence of years with Martin.. We all owe him a debt of gratitude for bringing to us what will be remembered as the Martin O'Neill years." (The Sun 2nd June 2000). The speculation will now continue as to whom will replace O'Neill. Candidates suggested by the media include Peter Taylor (Gillingham), Peter Reid (Sunderland), Dave Bassett (Barnsley), David Moyes (Preston), Joe Kinnear (ex-Wimbledon), Steve Walsh (Leicester defender), Steve Bruce (Huddersfield) and former Arsenal and Norwich manager Bruce Rioch

Both John Robertson and Steve Walford followed Martin to Celtic whose 2000/01 season started favorably including a 6-2 victory over Rangers on 27th August 2000. The Scottish League Championship was secured on 7th April 2001 in addition to the Scottish League Cup. The Green half of Glasgow are celebrating O'Neill's tenure at Parkhead. May 2001 saw Martin linked strongly as Alex Ferguson's replacement at Old Trafford when Fergie retires in May 2002. Martin became the Bookies favourite to succeed Fergie whilst the Sunday media alleged that representatives from the Theatre of Dreams had already spoken to O'Neill's agents. Fergie himself backed O'Neill saying "He is a no-nonsense manager who is strong and has a personality - these qualities are important at a big club. .. Any recognition and praise that Martin gets is deserved. For him to go to Celtic and change it all around in one season is remarkable". Celtic completed a historic treble beating Hibs on 26th May to win the Scottish Cup.

Following another successful season in 2001/02 that resulted in another Scottish Championship, O'Neill was linked with the vacancy at Elland Road following Leeds' sacking of David O'Leary on 27th June 2002. O'Neill was unable to comment as he was away in Japan commentating on the World Cup with the BBC. After ten days of speculation, O'Neill signified that he would stay at Parkhead, forcing Leeds to turn their attentions to Middlesbrough's Steve McClaren and Charlton's Alan Curbishley.

Speculation as to Martin's future arose again in January 2003 when he didn't accompany Celtic to their Florida training camp during the Scottish winter break. In addition to the much used link with Manchester United, he was rumoured to be heading for Merseyside to replace Gerald Houllier whose Liverpool side had faltered in recent months, going 11 games without victory. Martin was indeed in contract negotiations but with Celtic, agreeing a 12month rolling contract to follow on from the contract that was due to end in the summer of 2003. He was though linked with the vacancy at Villa Park in the summer of 2003 which eventually went to David O'Leary.

The 2002/03 season ended trophyless for the Hoops with arch-rivals Rangers completing a domestic treble. Celtic reached the final of the UEFA Cup only to lose to Porto with O'Neill facing disciplinary action following his outburst over the standard of refereeing.

In September 2003, Martin was suggested to be clear favourite to replace Glenn Hoddle at Tottenham following Spurs' poor start to the 2003/04 campaign and Hoddle's subsequent sacking following the 3-1 defeat at the hands of Southampton.

In March 2004, Martin was suggested to be on the English FA's shortlist to replace Sven Goran Eriksson if the Swede did not renew his contract after Euro2004. However many suspect this "leak" is a ploy to force Sven to sign.

Celtic won the Scottish Premiership for a third time under O'Neill on April 18th 2004. Martin was also named Scottish Manager of the Year for 2004

Martin left Celtic at the end of season 04/05, where Celtic gave the league to Rangers on the last day, after a 2-1 defeat to Motherwell. Unfortunate Martin left not by choice but because of the serious ill-health of his wife, understandbly so. So for season 05/06 Gordon Strachan is the new appointed Celtic manager.