He made his debut on 9 October 2004 in a 2–1 victory against Mile Oak Rovers,[7] only coming into the first team as several players were unavailable because the game was an early kick-off – it had been brought forward to avoid clashing with England's 2006 World Cup Qualifier against Wales.

He had been told he was to leave Walsall earlier in the summer, and had slackened his pre-season training in protest at the club's hardline stance when trying to sell him.

[5] Walsall also negotiated a 20% sell-on fee for any profit were Watford to sell him in the future, something that later would complicate potential transfers away from the club according to Deeney in 2018.

[18] At the beginning of the 2011–12 campaign, Deeney initially found it hard to acquire a starting spot in the team as many of his early season matches saw him consigned to a bench role.

[5] He soon found his way into the starting eleven and amassed a total of 46 appearances under the management of Sean Dyche in his preferred position as striker.

[35] Following the departure of Manuel Almunia, Deeney was named the Watford captain by manager Giuseppe Sannino prior to the 2014–15 campaign.

[38] On 8 August 2015, Deeney made his Premier League debut, captaining Watford in their 2015–16 season opener, a 2–2 draw with Everton at Goodison Park.

[44] He opened his 2016–17 goalscoring account on 10 September at West Ham United's new home venue, the London Stadium, equalising in a 4–2 comeback victory.

[45] Deeney scored his 100th Watford goal in all competitions on 26 December 2016 with a penalty in a 1–1 home draw against Crystal Palace, ending a ten-match goalless run.

[47] In April 2019 Deeney (and Watford team-mates Adrian Mariappa and Christian Kabasele) were subjected to racist comments on social media.

[48] In May 2020, following the Premier League's suspension due to the COVID-19 pandemic, Deeney initially refused to return to non-contact training with his teammates.

He said he did not wish to put his five-month-old child at risk, who has breathing difficulties, saying "it only takes one person to get infected within the group and I don't want to be bringing that home.

"[49] Deeney eventually returned to training and captained Watford in their first game back against Leicester City, playing the full 90 minutes.

On 11 July, he scored two second-half penalties to come from behind and win at home against Newcastle United, giving Deeney his first goals of the restarted season.

[50] Deeney revealed after the game he had been suffering from a right knee injury since the restart, but vowed to see out the end of the season as club captain and try to save Watford from relegation.

[54] Despite widespread rumours of his impending departure[55] and reported interest from Tottenham Hotspur,[56] West Bromwich Albion[57] and Fenerbahçe,[58] Deeney remained with Watford throughout the summer transfer window and came off the bench during their 1–0 win over Luton Town on 26 September 2020.

[60] Deeney came on as a 79th-minute substitute for Tom Cleverley in Watford's first match since their return to the top flight on 14 August 2021, helping secure a win after beating Aston Villa 3–2 at home [61] Deeney made his final appearance for Watford as a 78th-minute substitute, replacing Ken Sema in a 2–0 away loss to Brighton & Hove Albion on 21 August.

[62] On 30 August 2021, Deeney left Watford as a free agent after eleven years with the club; he had made 419 appearances for the team and scored 140 goals.

[72] Prior to joining Forest Green, Deeney confirmed he had received an offer from Australian club Melbourne Victory.

[73] On his debut two days later, he came on as a substitute in a 3–0 home loss to Newport County,[74] and on 26 August he came from the bench and scored the equaliser in a 1–1 draw at AFC Wimbledon.

[76] After manager David Horseman left the club by mutual consent, Deeney was appointed head coach on a permanent basis on 20 December 2023, at which time the team were 23rd in League Two, five points from safety.

[78] Following a 2–0 loss to Harrogate Town in January 2024, Deeney publicly criticised his squad, stating that he said that he would "rather watch Antiques Roadshow" than his team, in addition to singling out right-back Fankaty Dabo as "awful."

Hours before his dismissal, he had been given a four-match ban and fined £1,500 by the Football Association for his conduct in a defeat to Swindon Town on 29 December.

[5] On 25 June 2012, Deeney was sentenced to ten months' imprisonment after pleading guilty to a charge of affray, having attacked a group of students outside a nightclub.

[90] In October 2020, Deeney began working as a guest writer for The Sun, penning a fortnightly column for the paper's sports section.

He is also a weekly guest on TalkSport's Monday morning breakfast show, appearing alongside Laura Woods and Ally McCoist.