After playing for Queen's Park and Dundee United in Scotland, he joined Polish club Lech Poznań in 2013, where he won the 2014–15 Ekstraklasa title.

Douglas enjoyed a successful season with Wolves, winning the 2017–18 EFL Championship and making his full international debut.

Douglas was born in Glasgow and raised in the city's Pollok district.Douglas was educated at Mosspark Primary School and Rosshall Academy where he met future Dundee United teammate Scott Allan[4][5] He played for Livingston's youth teams, but was released at the age of sixteen because he was considered "too small".

He made his senior debut in August 2008 in a Scottish Second Division match against Stranraer, and went on to become a first team regular for Queen's Park, playing for the amateur club part-time while also working as a refrigeration engineer and serving an apprenticeship in the trade.

He spent three seasons at Tannadice; the player signed by United to replace him in the position, Andrew Robertson, also moved from Queen's Park, leading to disputes between the clubs as Dundee United had used the amateur status of Queen's Park and Douglas to avoid paying a transfer fee, and attempted to do the same with Robertson.

[16] With Konyaspor, Douglas reached the 2016–17 Turkish Cup final and played the full game as they defeated İstanbul Başakşehir on penalties.

[23] Douglas picked up a winner's medal during the 2017–18 EFL Championship with Wolves winning the league, contributing five goals and 14 assists from the left back position.

[25] On 28 July 2018, EFL Championship side Leeds United signed Douglas for an undisclosed fee (roughly £3 million) on a three-year contract.

[26] He made his debut and got an assist in the opening game of the 2018–19 season on 5 August 2018 against Stoke City at Elland Road in a 3–1 win.

[32] He started the season as Leeds' first choice left back, but after a series of injuries, he found himself out of the side behind Gjanni Alioski.

After an impressive half time substitute appearance against West Bromwich Albion on 1 January 2020, head coach Marcelo Bielsa said Douglas would start against Premier League side Arsenal in the third round of the F.A.

[33] Douglas played the full 90 minutes of the cup tie, which Arsenal won 1–0[34] but was substituted for Alioski during Sheffield Wednesday's late win at Elland Road on 11 January.