Although regarded as one of the best African players at the time, McCarthy never established himself as a regular choice for Celta's manager, Víctor Fernández.
At Porto, McCarthy played under the then newly appointed coach José Mourinho for an underperforming team that since winning the European Cup in 1987 had never been quite good enough to challenge for the top honours in Europe.
He helped them to third place in the Primeira Liga and automatic qualification for the UEFA Cup by scoring an impressive 12 goals in 11 matches, but Porto's finances did not allow them to keep the player, despite the desire of both sides to continue.
In 2002–03, McCarthy therefore returned to Celta, where he spent much of his time on the substitutes' bench as a squad player as Porto captured the Taça de Portugal, the Portuguese League and the UEFA Cup.
The move meant that Rovers regularly fielded a side that contained two South African Internationals, the other being Aaron Mokoena who joined the club the year previous.
After a disappointing performance in the side's 3–0 defeat to Portsmouth, McCarthy found the net on his debut at Ewood Park against Everton on 23 August 2006.
McCarthy was out of action for a few weeks and found first team opportunities limited, largely because the form of new striking arrival Roque Santa Cruz.
In the 2008–09 season, McCarthy appeared to be out of favour with new manager Paul Ince, as the club's strike force was strengthened with the arrivals of Carlos Villanueva and Robbie Fowler to join the already established Roque Santa Cruz, Jason Roberts and Matt Derbyshire.
He made his Premier League debut for West Ham against Burnley at Turf Moor in a 2–1 defeat on 6 February 2010, where he sustained an injury which would keep him out for six weeks.
[15] After leaving West Ham, McCarthy trained with former club Ajax Cape Town during the ABSA Premier League off-season.
McCarthy scored two goals in the final half-hour to help the Pirates defeat the Golden Arrows 4–2 on 19 May 2012 to retain their Premier League title.
[18] McCarthy, who is affectionately known as "Big Brother" by the Orlando Pirates faithful, was handed a red card for dissent in the MTN 8 secondleg semi-final played on 25 August 2012 after he charged Franklin Cale after a reckless challenge on Daine Klate.
Along with veteran Egyptian striker Hossam Hassan, McCarthy was the joint top scorer in the 1998 African Cup of Nations in Burkina Faso with seven goals, including four in 13 minutes against Namibia.
The 1998 FIFA World Cup in France was the first time South Africa's national football team had reached the later stages of the competition, and McCarthy was one of their key players.
After a 2–2 draw against Paraguay and a 1–0 victory over Slovenia, McCarthy's 30th-minute equaliser in the last match against Spain was not enough, as South Africa eventually lost 3–2 and finished with another third place in the group stage.
"[23] After struggling with his fitness and facing criticism in his nation for being overweight, McCarthy was omitted from the final South African 23-man squad for the 2010 World Cup, which was announced on 1 June 2010.
[24] Living in Edinburgh since 2013, McCarthy confirmed in April 2015, that he had joined the coaching staff of Alan Stubbs at Scottish club Hibernian.
[27] On 13 June 2017, McCarthy was unveiled as the new Cape Town City head coach,[28] replacing Eric Tinkler, who moved to manage SuperSport United.
He took the club to a second-place finish in the DStv Premiership, a record high for AmaZulu in the modern era of the game in South Africa.
The song released in the run-up to the 1998 World Cup in France samples greatly on "The Final Countdown" by Europe and features McCarthy rapping part of the lyrics.