[3] O'Mahony won the 2009–10 All-Ireland Cup with Cork Constitution and was named player of the match in the final.
[5][6][7] O'Mahony made his Munster debut against Ulster on 2 January 2010, coming on as a replacement for James Coughlan.
He made his Heineken Cup debut for Munster against Northampton Saints on 12 November 2011,[12] a game in which he won the Man-of-the-Match award.
[16] O'Mahony started for Munster in their opening fixture of the 2012–13 Heineken Cup on 13 October 2012, a 22–17 defeat at the hands of Racing 92.
[23] O'Mahony continued to start for Munster in the knockout-stage of the 2012–13 tournament, playing a crucial role in their 18–12 away quarter-final victory against Harlequins on 7 April 2013.
[25] O'Mahony was named as the new Munster captain on 30 July 2013, replacing former All Black Doug Howlett, who retired at the end of the 2012–13 season.
[26] He started for Munster against Edinburgh in the opening round of the 2013–14 Heineken Cup on 12 October 2013, captaining the side during their 29–23 defeat.
[32] O'Mahony started in Munster's 47–23 Heineken Cup quarter-final win against Toulouse on 5 April 2014, but went off with a shoulder injury in the 18th minute.
[44] O'Mahony made his comeback from the knee injury he suffered with Ireland during the 2015 Rugby World Cup on 1 October 2016, coming on as a replacement for Billy Holland in the 2016–17 Pro12 fixture against Zebre.
[45] On 1 April 2017, O'Mahony was captain in Munster's 41–16 2016–17 European Rugby Champions Cup quarter-final victory against Toulouse, though he went off during the second-half with a leg injury.
Despite this, Munster won the game 30–27, and a Pro14 judicial officer subsequently ruled that the two yellow cards were sufficient and no ban was necessary.
[56] He signed a two-year contract extension with the IRFU in March 2021, a deal that will see O'Mahony remain with Munster until at least June 2023.
[58] Having missed the first leg of Munster's 2021–22 Champions Cup round of 16 fixture away to English club Exeter Chiefs due to injury, which the hosts won 13–8, O'Mahony returned for the second leg one week later on 16 April 2022 to lead from the front and earn the player of the match award in the province's 26–10 home win, which secured a 34–23 aggregate win and a record-equallinging 19th Champions Cup quarter-final for Munster.
The match was tied 24–24 after extra-time, and Toulouse won the penalty shootout 4–2, but O'Mahony's performance during the match, which included four turnovers and a huge defensive effort, garnered widespread praise, with both Munster head coach Johann van Graan and O'Mahony's former British & Irish Lions teammate Sam Warburton calling him a "warrior".
[60][61][62][63] O'Mahony signed a one-year contract extension with the IRFU and Munster in February 2023, meaning he will remain with the province for the 2023–24 season.
[75][76][77][78][79][80] He also featured in the 2013 Ireland tour to North America,[81] and was made captain after Rory Best was called up to the 2013 British & Irish Lions squad.
[84] O'Mahony featured in four of Ireland's games, picking up a Man-of-the-Match award against Wales, in their championship-winning 2014 Six Nations.
[103] On 12 November 2016, O'Mahony made his return to the senior Ireland team, captaining them in the 52–21 win against Canada.
At the end of the game against England, O'Mahony gave his winner's medal to an Irish supporter with Down syndrome.
[132] Retained by new head coach Andy Farrell in his squad for the 2020 Six Nations Championship, O'Mahony featured off the bench in Ireland's 19–12 opening win against Scotland on 1 February 2020,[133] then started in their 24–14 win against defending champions Wales on 8 February,[134] and their 24–12 defeat against England,[135] before the tournament was suspended due to the COVID-19 pandemic.
[138] With the usual format of end-of-year international tests not possible due to the COVID-19 pandemic, Ireland instead participated in the Autumn Nations Cup.
[145] During the 2021 July rugby union tests, O'Mahony started in Ireland's 39–31 win against Japan[146] before commencing his off-season.
[159] During the 2022 Autumn Nations Series, O'Mahony started for Ireland in their 19–16 win against world champions South Africa on 5 November.
[182][183] Later that month, he announced he would be retiring from international duty alongside teammates Conor Murray and Cian Healy following the end of the tournament.
[184] In April 2017, O'Mahony was selected in Warren Gatland's 41-man squad for the 2017 British & Irish Lions tour to New Zealand.
[185] On 7 June 2017, O'Mahony made his non-test debut for the Lions when he came off the bench against the Blues in Eden Park.
[190] O'Mahony played for 64 minutes, before being replaced by tour captain Sam Warburton, and led the side to a record 32–10 victory against the Māori.
[192][193] He became the 11th Irishman to captain a British & Irish Lions test team and the first Cork Constitution player to do the same since Tom Kiernan in 1968.