Fer had his breakthrough in Feyenoord’s first team in the season 2007–08. On 2 December 2007, he made his official debut for Feyenoord under the management of Bert van Marwijk. At age 17, Fer replaced Nuri Şahin in the 84th minute in the Eredivisie home match against Heracles Almelo (6–0). Four days later, on 6 December 2007, Fer signed his first professional contract with Feyenoord, keeping him at De Kuip until summer 2012. Fer was active in 13 Eredivsie matches in his first season, mainly coming in as a substitute. Due to injuries in Feyenoord’s first team squad, Fer made his first appearance in the starting line-up in the Eredivisie away match against Ajax (3–0) on 3 February 2008. Fer started on the right back position, which he continued to do so for four matches in a row. On 30 March 2008, Fer scored his first Eredivisie goal in the away match against NAC Breda (3–1).

In the 2008–09 season, Fer slowly became a first team regular. New Feyenoord coach Gertjan Verbeek often positioned Fer on the central attacking midfielder position behind striker Roy Makaay. On 18 September 2008, Fer made his official European debut, as he was named in the starting line-up in the UEFA Cup home match against Kalmar FF. Feyenoord lost the match 0–1, but in the return match in Sweden, Fer scored the winning goal in the 1–2 victory, securing a place in the group stage. During the season, various European top clubs showed interest in the Feyenoord talent, including Juventus, but Feyenoord refused any cooperation.

Despite playing for the Netherlands on various youth levels, Fer was in doubt which country to represent, as he was eligible to play for both the Netherlands and Netherlands Antilles. In October 2008, Fer claimed he had not made a decision yet: “Personally I want to mean something for my island. That’s why I’m still in doubt. My brain says it’s better to choose for the Netherlands. Then you’re assured of a big tournament every two years. But my heart is thinking of Curaçao. It would be fantastic to play a World Cup with Netherlands Antilles, it would give the islands an enormous boost. My roots are stronger than my urge to a career, I’m striving for a higher cause. I don’t know yet, but my feelings to play for Netherlands Antilles are very strong.” However, on 26 August 2009, Fer announced he made the final decision and chose to represent the Netherlands at international level.

For the 2009–10 season, Fer lost his status as talent and grew out to be an important player. While he wandered through various playing positions in the previous seasons, from right back to striker, Fer was now constantly placed on the defensive midfielder position by new Feyenoord coach Mario Been. On 20 October 2009, Feyenoord’s new technical director, Leo Beenhakker, criticized Feyenoord’s first team squad, but at the same time praised Fer for his importance within the team. Three days later, Feyenoord captain Giovanni van Bronckhorst even claimed Fer to be “unmissable” for the squad in Feyenoord TV, while Netherlands national team head coach Bert van Marwijk admitted he was following Fer’s development closely and was getting very optimistic about him for a possible call-up.

Fer made his unofficial debut for the Netherlands under-21 team on 31 March 2009. A temporary replacement team for the Netherlands, called the Netherlands B, played a friendly match against Italy, which ended in a 1–1 draw. On 11 August 2009, the Netherlands began their 2011 UEFA European Under-21 Championship qualifying campaign with a friendly match against England. Fer was part of the starting line-up in a match which ended 0–0.

On 28 January 2013, it was announced that English side Everton of the Premier League had agreed a deal, reportedly worth £8.6 million, to sign Fer. However, the deal fell through when a medical revealed the player had a long-term knee injury and Everton wanted to negotiate a payment by appearances fee.

On 13 July 2013, Norwich City confirmed the signing of Fer on a four-year deal for an undisclosed fee believed to be in the region on £4.4 million. He made his debut for Norwich against Hull City. He scored his first goal for the club against Bury in the League Cup on 27 August 2013. He was awarded Man of the Match for an exceptional performance in a 1–0 home victory over Southampton on 31 August. Fer continued his fine early form, and was awarded Norwich’s Player of the Month award and his fourth consecutive Man of the Match award after a 3–1 home defeat to Chelsea on 6 October.

Fer was involved in controversy in a 0–0 draw against Cardiff City on 26 October 2013. Teammate Alexander Tettey went down injured late in the match and Cardiff goalkeeper David Marshall sportingly put the ball out of play so Tettey could get treatment. However, during the distraction, Ricky van Wolfswinkel took a quick throw-in to Fer, who rolled the ball into the net. Despite the goal not technically being in violation of any rules, the referee disallowed the goal as he “had not blown his whistle”; players technically do not have to wait for the whistle to be blown to take a throw-in. Cardiff manager Malky Mackay criticised Fer’s lack of sportsmanship. Fer confirmed on Match of the Day that the shot was completely intentional and had not expected such a reaction from the Cardiff players, who surrounded him and started shoving him around. Marshall also started to threaten Van Wolfswinkel.

Fer scored the third goal in Norwich’s 3–1 home win over West Ham United on 9 November 2013, and drew plaudits from the likes of Jamie Redknapp for his performance. Redknapp also stated his belief that Fer could become one of the stars of the league if he consistently plays as he did.

Fer scored again in a 2–1 defeat against Newcastle United on 23 November, a late header that only proved to be a consolation for the Canaries. He scored his third league goal for the club against West Bromwich Albion in a 2–0 victory on 7 December, and celebrated by imitating a flying canary. He also assisted Gary Hooper with a through ball that cut straight through the West Brom defence. He was named Man of the Match by Sky Sports for his performance. He was voted Norwich’s player of the month for November via the club’s official Facebook and Twitter pages. He was sent off against Crystal Palace on 1 January 2014 in the 82nd minute in a match which ended 1–1.

He was selected for the Netherlands’ 23-man squad for the 2014 FIFA World Cup. He scored his first World Cup goal in the group match against Chile on 23 June 2014, after coming on as a substitute.

Following Norwich’s relegation to the Championship, Fer moved to newly promoted Premier League team Queens Park Rangers, signing a three-year deal for a reported fee of £8 million. Fer made his debut for the Hoops away against Tottenham Hotspur. The next league match was Fer’s home debut when Sunderland were welcomed to Loftus Road, where Fer was named Man of the Match for his performance in QPR’s first league win and clean sheet of the season. Fer scored his first QPR goal against Leicester City, smashing in a close-range effort in a 3–2 home win. Seven days later, Fer was again on the score sheet, scoring his second goal with the opener for QPR in a 2–0 win over Burnley at home. On 2 May 2015, Fer scored an equaliser against Liverpool at Anfield, only for the home side to score a late winner.

On 1 February 2016, Fer joined Premier League side Swansea City on loan for the remainder of the season after failing to make an impact under Jimmy Floyd Hasselbaink.

Fer made his move to South Wales permanent on 5 July 2016. He began his Premier League season with a winning goal in a 0–1 away victory at Burnley. The club confirmed on 18 May 2019 that he will be released upon expiry of his contract.

On 25 July 2019, Fer returned to Feyenoord after he had already been training with the club since 8 July.