Viðar made his debut for Selfoss in 2006, aged 16. He joined Icelandic top division club ÍBV in 2009 but rejoined Selfoss after a short stint with ÍBV.

He joined Icelandic club Fylkir in 2013 and after a very prolific goalscoring season in the Icelandic top flight, he joined Norwegian club Vålerenga in December 2013. Viðar scored his first goal for Vålerenga in a 3-1 home win over Bodø/Glimt on 6. April 2014.

Viðar made his senior international debut for Iceland in 2014, starting as a centre forward in a friendly against Austria. He scored his first international goal in a friendly against the United Arab Emirates in January 2016. Despite a strong run for his club team Malmö FF, Viðar was overlooked for a spot in Iceland’s Euro 2016 squad and instead put on the team’s standby list. His second international goal came in a friendly against Qatar in November 2017. Viðar once again missed out on playing in an international championship, when he was left out of Iceland’s World Cup 2018 squad, serving as a non-squad reserve.

On 22 January 2015, Viðar transferred to Chinese Super League side Jiangsu Guoxin-Sainty. Jiangsu Guoxin-Sainty signed him for €3.5 million. On 7 March 2015, he made his debut and scored his first goal for Jiangsu in the first league match of the 2015 season which Jiangsu Sainty lost to Shanghai SIPG 2–1.

On 27 January 2016, he signed for Swedish club Malmö FF.

On 30 August 2016, he transferred to Israeli club Maccabi Tel Aviv. He signed a four-year deal with the club. On 26 October 2016 Viðar won the Swedish championship as Malmö FF sealed the title in his absence.

On 31 August 2018, he signed a four-year contract with the Russian Premier League club FC Rostov, joining his compatriots Ragnar Sigurðsson, Björn Bergmann Sigurðarson and Sverrir Ingi Ingason at the club. He scored his first two goals for the club on 26 September 2018 in a Russian Cup 4–0 victory over a third-tier club FC Syzran-2003.

Viðar announced his retirement from international duty in October 2018 at the age of 28, having won 19 caps and scored 2 goals.

On 18 March 2019, Viðar signed a five-month loan deal with Hammarby IF in Allsvenskan.

On 20 July 2019, he joined Russian club FC Rubin Kazan on loan for the 2019–20 season. He played 16 league games and scored one goal during the first half of the season, as Rubin Kazan was placed 13th in the table, before terminating his loan deal on 11 January 2020. On 27 January 2020, Rostov announced that he was loaned to Turkish club Yeni Malatyaspor.

In March 2019, Viðar reversed his decision to retire from the national team and was called up to represent Iceland in the UEFA Euro 2020 qualifying games against Andorra and France.

On 28 August 2020, he returned to Vålerenga and signed a contract until 2023.