He was a key part of the Leeds United team under Marcelo Bielsa that won the 2019–20 EFL Championship and promotion to the Premier League.

[11] On 12 April 2012, it was announced that Dallas had signed a pre-contract with League One side Brentford who were managed by Uwe Rosler and he joined the club that summer.

[18][19] He made his debut for the Bees (the first professional appearance of his career) in a 1–0 Football League Trophy win over Crawley Town on 9 October, coming off the bench after 75 minutes to replace Scott Barron.

[20] After making 3 cup appearances for Brentford in the 2013–14 season,[23] Dallas signed for League Two side Northampton Town on a month's loan on 5 October 2013 and went straight into the squad for the Cobblers' match that day against AFC Wimbledon.

[24] He bagged his second Northampton goal in his third match for the club, following up fellow Brentford loanee Luke Norris's opener in a 3–2 defeat to Rochdale on 22 October.

[27] An injury suffered to Brentford's on-loan Cardiff City winger Kadeem Harris meant that Dallas was recalled early from his loan on 23 December.

[39] In the following game, Dallas was pressed into service as an emergency right-back for the final 25 minutes of a 3–1 win over Nottingham Forest, having replaced the injured Moses Odubajo.

[46] On 31 July 2015, the Yorkshire Evening Post reported that Brentford had accepted a bid for Dallas of over £1m from fellow Championship side Leeds United.

[51] He made his debut for Leeds in the Football League Championship in a 1–1 draw against Burnley on the opening day of the season,[52] and scored his first goal for the club against Wolverhampton Wanderers on 17 December 2015.

[58] After missing part of pre-season training for the 2016–17 season due to his participation in UEFA Euro 2016 with Northern Ireland,[59] Dallas started Leeds' first match of the season as they lost 3–0 away to Queens Park Rangers,[60] but suffered a calf injury whilst on international duty with Northern Ireland in October 2016.

[62] Due to injuries to several fullbacks, including Luke Ayling and Laurens De Bock, Dallas was used as a right-back for periods during the 2017–18 season.

[74] Dallas found himself one of the first names on the first team sheet, being ever present for the first half of the season, playing in a range of positions, including left-back, right-back and central midfield after injuries to Adam Forshaw, Jamie Shackleton, Tyler Roberts and Pablo Hernández.

[77] Dallas made his Premier League debut in the first match of the season against Liverpool on 12 September 2020, starting in the 4–3 defeat at Anfield to the reigning champions.

[78][79] He scored his first Premier League goal on 2 November 2020 in a 4–1 home defeat to Leicester City after his cross "flew over everybody in the box and into Schmeichel's net".

[82][83] On 10 April 2021, he scored both goals as Leeds won 2–1 away at Manchester City despite playing the second-half with ten players following a red card for Liam Cooper.

[84] Manager Bielsa praised his performance, stating that it gave him "great happiness" because his game is based on "a lot of generosity" adding that "this year has allowed Dallas to show a part of himself that we didn’t know he had.

[91] On 10 April 2024, Dallas officially announced his retirement from football, stating that he "must now accept the fact [his] knee suffered irreparable damage".

[91][92] Due to his form for Crusaders early in the 2010–11 season, Dallas (along with team-mates Chris Keenan, Martin Donnelly and Jordan Owens) received a call-up to the Northern Ireland U23 squad for an International Challenge Trophy match versus Portugal U23 on 12 October 2010.

[95] Early in the 2011–12 season, Dallas received a call into the Under-21 side for a 2013 UEFA European Under-21 Championship qualification match against Denmark on 6 September 2011.

[57] Dallas came on as a half time substitute for Paddy McNair in Northern Ireland's 1–0 loss in their opening UEFA Euro 2016 game against Poland on 12 June 2016.

[101] On 16 June, he started in their 2–0 victory over Ukraine and his parried shot fell to Niall McGinn to score Northern Ireland's second goal in the sixth minute of second-half stoppage time.

[11] Dallas started both legs of the two-legged tie against Switzerland as Northern Ireland lost 1–0 on aggregate and missed out on qualification to the World Cup.

[6] After getting engaged in 2013, Dallas married his long-term partner Juneve Lamont in June 2016, having been in a relationship with her from the age of 16 when they both attended Cookstown High School.