[6][3] He was increasingly involved with the first team,[5] and on 3 October 2004, after Franck Queudrue failed a late fitness test, the 18-year-old McMahon made his senior debut in the starting eleven for a 1–1 Premier League draw with Manchester United at Old Trafford.

[3][7] He made a European debut on 21 October in the UEFA Cup group stage away win at Egaleo,[8] and continued in the team well into December, mostly as a starter, taking his appearance tally to 14.

[9] His performances earned plaudits from Match of the Day pundit Alan Hansen,[10] Middlesbrough team-mate Ray Parlour,[11] and manager Steve McClaren, who said that "If anyone can handle Old Trafford in the way in which Tony did on his debut, then we know that he can be relied on in the future.

"[12] After Boro's injury crisis eased with the return of more established defenders in Michael Reiziger and Stuart Parnaby, McMahon played little in the second half of the season.

[14] In mid-March, he started in the 1–0 away defeat against Sporting CP in the last 16 of the UEFA Cup that eliminated Middlesbrough from the competition,[15] but suffered knee damage that required surgery and six weeks' rehabilitation.

He dislocated a shoulder during a reserve match in August, returned to reserve-team action five months later, and did not appear for the first team for another four weeks, when he played the first half of a fifth-round FA Cup tie.

[18] He lasted less than ten minutes of the Premier League visit to Charlton Athletic before a poor tackle by Bryan Hughes inflicted medial ligament damage to his knee that kept him out for another six weeks, and he played just twice more at the end of the season.

[19][20] Under the management of former Middlesbrough defender Gareth Southgate, McMahon was one of several young players with first-team experience who struggled for game time in the first few weeks of the 2006–07 season.

He returned to reserve-team action after four months,[23] and was included on the bench for a Premier League match in April,[24] but an injury sustained in training, initially described as "a knock" that was not thought serious,[25] but in fact another broken leg,[26][27] not only put an end to his comeback but also delayed the start of his 2007–08 campaign.

[38] He was more or less a regular in the side as injury cover, for Emmanuel Pogatetz and Andrew Taylor as well as Hoyte, as the team faced an ultimately unsuccessful struggle against relegation to the Championship.

[42] Selected ahead of Hoyte by Southgate to start the opening match of the 2009–10 season, a goalless draw with Sheffield United, McMahon's performance earned him a place in the Football League Team of the Week.

[55] He quickly established himself in the promotion-chasing side,[56] and started every match as they remained unbeaten in the League until mid-November,[45][57] when hamstring problems meant he only played an hour's football in the next month.

[67] Apart from a two-match suspension,[68] he remained in the starting eleven,[45] contributing to the team narrowly avoiding relegation and earning himself the Wonga Player of the Month award for March.

[70] Under McMahon's captaincy, the team achieved a first win on 6 October against Cardiff City,[71] He scored his first goal in the next match, a defeat to Huddersfield Town, and received his fifth yellow card in the next.

At the time, Blackpool led 2–0, but with only ten men were unable to retain the lead, and manager Lee Clark was angered and disappointed at his captain's behaviour.

[76][77] He made his debut the next day as a late substitute, scored his first goal the following week, and started the last four matches of the season, in which Bradford City remained unbeaten.

[80] He lost time in the early part of the season to a virus, but was soon able to establish himself in the side, despite his right-back position of choice being occupied by captain Stephen Darby.

[87][85] He continued in the starting eleven, and scored twice in the penultimate match of the season, a 3–0 win against AFC Wimbledon that confirmed Bradford City's qualification for the play-offs.

[90] With his contract due to expire and amid interest from clubs including Bolton Wanderers, Blackburn Rovers and Preston North End, McMahon delayed accepting an offer to stay with Bradford City, but in early July, he signed for another year.

[99] He started six matches in October, in which he helped the side keep three consecutive clean sheets,[45] before a cracked rib, aggravated by playing on for 15 minutes after sustaining the injury, put an end to his Oxford United career.

[107] Still hoping to find a club nearer home,[101] McMahon remained with Oxford United during pre-season, but was released from his contract by mutual consent on 3 September 2019.

[111] McMahon departed his role as assistant manager alongside the sacking of Keith Hill on 30 August 2022 with the club sat second bottom of the National League.

[121] McMahon was born in Bishop Auckland, County Durham,[5] and lived in nearby Evenwood with his parents, Tony and Tracey, a sister and a brother.

[138] Early in his Middlesbrough's career, McMahon launched Future Goals, which is a new football and fitness programme for children and their parents to improve their lifestyles.