Willo came through the youth system at Cherry Orchard in Ireland before joining the academy at Manchester City. Between 2002 and 2006, he made occasional appearances for the Man City first team while also spending time on loan at Coventry City and Rochdale. A Republic of Ireland youth and Under-21 international, he moved to Cardiff City in 2006, where he spent three seasons. It was during this time that he was brought to United on loan for the 2007-08 season under Craig Levein. During this season, he played in all bar two matches, collecting a runners-up medal and the man of the match award in the League Cup Final against Rangers at Hampden Park.
United retained his services for the final year of his Cardiff contract, once again on loan, however in January 2009, Cardiff City sold him to Celtic, ending his time at United. Willo was with Celtic for a year before he moved to Middlesbrough to join former manager Gordon Strachan, where he was a first team regular until injury hampered his time at the Riverside Stadium. He returned to United in May 2011, following his release from the Riverside club, signing a two year deal at Tannadice.
Willo became a key player in Peter Houston’s first team during the 2011-12 and 2012-13 seasons, earning the Club’s Player of the Year award in 2013. Known more for his work rate and combative style than his goal scoring, Willo has scored some stunning goals during his career, including a goal versus St Mirren in 2007 that won him the SPL Goal of the Season award, and a cracker in the Europa League against Dynamo Moscow.
After more than 150 appearances with United, his contract expired at the end of the 2012-13 season, at which time he moved on to Aberdeen. He spent three seasons at Aberdeen as a first team regular, winning the League Cup in 2013-14, before he was released in July 2016. Following United’s relegation to the Championship, Flood re-joined the Terrors for a fourth time during the 2016-17 pre-season.