He was capped nine times for the Norwegian under-17 side and then made two appearances for the under-21s in 1996. Deila never represented his country at senior level.

Deila began his playing career with lower league club, Urædd. He then joined Odd, where he became a mainstay in their defence. In 2000, he played in the side that defeated Viking 2–1 in the Norwegian Cup Final, his only major honour as a player. He joined Viking in 2004, before moving on to join Strømsgodset as player/assistant coach in 2006.

After leaving Viking, Deila coached Norwegian fourth-tier club Brodd briefly in 2005. He then joined Strømsgodset in 2006 as player/assistant coach, working under head coach Dag-Eilev Fagermo until the end of the 2007 season, when Fagermo decided to move to Odd.

Although by 2009 he was coaching Strømsgodset, he combined these duties for a couple of years with playing part-time at lower league Sparta Bragerøen until 2011.

His first two seasons as head coach saw the club struggle against relegation, but his attacking philosophy began to bear fruit as Strømsgodset started to record higher finishes in the Norwegian league. In 2010, he won the Norwegian Cup, and in 2013 he won Tippeligaen with Strømsgodset, their first title in 43 years. The same year, Deila was also awarded the Kniksen Award for Coach of the Year.

Deila showed a keenness to take on board coaching ideas at foreign clubs. Having previously visited Manchester City, Barcelona and Ajax to acquire further knowledge, he then after the end of the season in 2013 travelled to Borussia Dortmund and Liverpool to study their coaching and training methods.

In the 2014 season, Deila’s Strømsgodset were second in the league by the end of May after eleven games. The club had continued their impressive home record under Deila, having gone 44 league games unbeaten at their Marienlyst Stadium since June 2011; just one short of the all-time Norwegian record held by Rosenborg. However, Deila was beginning to attract foreign attention, and Strømsgodset had given him a new contract to run until 2016 in order to head off interest from Swedish champions Malmö who had unsuccessfully attempted to poach him in January. Despite this new contact, various media reports surfaced in the first week of June linking Deila to the vacant managerial post at Scottish champions Celtic.

Deila was appointed manager of Scottish Premiership club Celtic on 6 June 2014. He signed a 12-month rolling contract with the club, describing his appointment as a “magnificent honour” before stating his desire to deliver “attacking, exciting and entertaining football.” Eleven days later, former Celtic and Scotland midfielder John Collins was appointed assistant manager. Deila described Collins as a “first-class coach” and stated that he “has ideas on football which are very similar to mine so I am sure he will be a great addition to my team.”

After going unbeaten on a pre-season tour in Austria, Deila’s first competitive match as manager of Celtic, a Champions League qualifying tie away against KR Reykjavik on 15 July 2014, ended in a 1–0 win for the Scottish club. The second leg of the tie ended in a 4–0 win for Celtic, securing a 5–0 aggregate win. The following qualifying round saw Celtic lose 6–1 on aggregate to Legia Warsaw. Despite this, Celtic were given a reprieve when it was discovered that Legia had fielded an ineligible player in the second leg. UEFA punished the Polish club by awarding the game 3–0 to Celtic, levelling the aggregate score at 4–4 and seeing the Scottish champions progress on away goals. Celtic went on to face Maribor in the next qualifying round. After an 1–1 draw away in the first leg, Celtic lost 1–0 in the return match at Celtic Park and went out on aggregate; dropping down to the Europa League.

Deila’s Celtic team won the Scottish Premiership on 2 May 2015, with three games to spare, following their rivals Aberdeen’s loss to Dundee United. Celtic finished the season in style, as Ronny’s Celtic side beat Inverness 5–0 on 24 May 2015.

In August 2015, Celtic failed to progress from the UEFA Champions League playoffs, having failed to overcome Malmö, and were consigned to playing in the Europa League that season. In November, Celtic exited the Europa league, having finished bottom of their group. On 17 April 2016 Celtic played Rangers, in a Scottish Cup semi-final at Hampden. The match went to extra time before Celtic lost the match on penalties. The way in which Celtic lost the match was also subject to criticism from the media and the Celtic support as Rangers dominated large portions of the game. Three days after the defeat and following intense speculation, Deila announced that he would step down at the end of the season. Celtic clinched the Scottish Premiership title following a win over nearest challengers Aberdeen on Sunday 8 May.

On 13 July 2016, Deila signed a four-year contract with Vålerenga, starting 1 January 2017. Vålerenga spent most of Deila’s first season struggling just above the relegation places in the league. His predecessor, Kjetil Rekdal, described the team’s season as a “failure” and expressed concern they may be relegated. Football analyst and former player Bernt Hulsker described the team as being “exceptionally weak”. The side eventually finished mid-table, in eighth place, still far below Deila’s expressed aim at the start of the season of a top-three place.

On 6 January 2020, Deila was appointed head coach of New York City FC of Major League Soccer.