Liverpool-born Flanagan became one of the youngest players in Liverpool’s history to be allocated a first-team squad number when he was given the No. 38 shirt for the 2010/11 season when still several weeks short of his eighteenth birthday. Like his fellow defender Jack Robinson, Flanagan had impressed in Rodolfo Borrell’s Under-18 team at the Academy. Tony Barrett, who was then at the Liverpool Echo, predicted big things for the young Liverpudlian as the step-up to the reserves did not phase Flanagan in the slightest and he "just got on with the job in hand with the minimum of fuss, defending with a characteristic zeal but never missing the chance to get forward and get crosses into the box whenever the opportunity presented itself." Flanagan made the sort of impact on his first-team debut that all young players must dream about. The right-back played from the start against Manchester City at Anfield in April 2011 with great confidence and maturity, as he did only six days later in an equally tough encounter with Arsenal in London. At the Emirates, he accidentally knocked out his captain, Jamie Carragher, but did not let that affect the level of his performance. He then played the full 90 minutes against Birmingham followed by 80 minutes against Newcastle. At Fulham in early-May, he was asked to play on the left so that Glen Johnson could be accommodated on the right. Despite being in a position that he was not accustomed to, he still played with great assurance. To play so well in the final seven Premier League matches of the season was very commendable and it is hard to believe that he only became eighteen in January 2011.
Despite the positive impact he made following his introduction into the first team towards the end of the 2010/11 season, Flanagan only made eight senior appearances in 2011/12. Flanagan played at Blackburn in April, but didn't complete the match because already cautioned, he was possibly fortunate not to be dismissed for another foul and was then substituted after his error compelled Doni to concede a penalty, as a result of which the goalkeeper was sent off. Rodgers relegated Flanagan to the reserves for a spell in 2012/13 and was disappointed to figure only twice for the first team. He started just once, away to Anzhi Makhachkala in the Europa League in November and came on as a substitute in the FA Cup-tie at Mansfield in January.
Flanagan made a massive breakthrough in 2013/14 by appearing in more first-team matches (25) than his total for the previous three seasons (17). The unbridled way in which he celebrated his first Liverpool goal at Tottenham in December showed how much playing for his hometown club means to him. Starting in seventeen matches after the turn of the year showed how much faith the manager had in him. Although predominantly right-footed Jon has usually been asked to play on the left but still performs there as well. His consistently good performances brought him to the attention of the national team manager and the full-back received a massive boost by being named as one of the seven stand-by players for the World Cup finals in Brazil. It surely can only be a matter of time before Jon Flanagan becomes a regular member of the national squad as well as being a first-team regular for his club.
Flanagan's 2014/15 season was a complete write-off. A series of injuries meant that he was unable to follow up the twenty-five first-team appearances he had made in 2013/14. In September he had corrective surgery on his left knee. Although he was able to play for thirty minutes in a charity match at Anfield at the end of March more surgery was required in April that would keep him out of action for several more months. In May he signed a new contract which made it clear that the club still saw him as an important part of its future once he had made a full recovery from his injuries. Flanagan made his long-awaited return after being 619 days out, when coming on against Exeter in the 3rd round of the FA Cup on 20 January 2016, prompting a memorable reception for the local lad. The following week he played 115 minutes of a League Cup semi-final with Stoke when he had only been expected to play for an hour. Klopp's faith in him was demonstrated when he was named captain for a match at Southampton on 20 March but his return to action was managed very carefully by Klopp. He was omitted from the Europa League squad and started just five games, the manager explaining that he wanted him to 'build a base for his fitness' and ensure he had sufficient recovery time between rigorous games.
In August 2016 Flanagan joined Premiership Burnley on a season-long loan but things didn't go as he would have hoped. He started just three league games and was an unused substitute 25 times, playing only 45 minutes of league football in the second half of the season. As 2017/18 started he was back to full fitness but omitted from the first two matchday squads and appeared to be third choice at both right-back and left-back. His only appearance in the first half of the season was at Leicester in the League Cup and he was an unused substitute twice. He also brought himself unwanted publicity when he assaulted his girlfriend in the early hours of 22 December 2017 in Liverpool city centre, leading to him appearing in court and being sentenced to community service. The club opted not to sack him, believing it would not help his rehabilitation. He was loaned to Bolton for the remainder of 2017/18, making nine appearances. On 8 June Liverpool confirmed that he would be released at the end of the month on the expiry of his contract and on 21 June he agreed a two year deal with Rangers.
Flanagan began the 2018/19 campaign as Ranger's first-choice right-back but he soon lost his place. He did however feature in Europa League games. He didn't make a single league appearance between January and March but did force his way back into the side at the beginning of April. He was further frustrated in 2019/20 by a groin problem that limited him to five league appearances and he left at the expiry of his contract. In September 2020 it was reported that he as close to agreeing a deal with French Ligue 2 side Amiens. That failed to materialise and on 4th November he agreed a short term deal with Belgian side Charleroi. This proved to be a disastrous move, as he tested positive for Covis 19 straight away and then had his contract cancelled after two months, having failed to make a single match day squad.
In the summer of 2021 Flanagan signed for Danish second tier side HB Koge, managed by former Red Daniel Agger, on a two year deal. After playing in the opening two league games of the season his back luck with injuries resumed with a foot problem and he also underwent further knee surgery. He managed just two more appearances and both parties terminated the contact by mutual consent at the end of the season. Following medical advice, Flanagan announced on social media on 18th October 2022 that he was retiring from playing, saying his long term health had to come first.