Career
He began his career with local club GÍ Gøta. After GÍ Gøta and Leirvík ÍF merged in 2008, he played for the newly formed club Víkingur. He scored 19 goals in the 2009 season, making him the league top goalscorer.
During the 2010 season he scored 7 league goals and 3 in the Faroese Cup.
In 2011 he was top scorer again, with 21 goals. Having previously been on trials with Swedish Allsvenskan clubs Örebro Saskatchewan and Trelleborgs FF, Finnur trialed with Superettan club Jönköpings Södra IF in late 2011.
After impressing on trial he signed a permanent deal with the club in early 2012. In 2013, he returned to Víkingur ahead of the 2013 season.
On 9 October 2015, Justinussen joined Danish 2nd Division-side Fremad Amager.
Justinussen made his U21 debut in September 2009, in a 1-1 draw with Moldova. He was called up to the Faroe Islands national team for the first time in September 2011, for the matches against Italy and Serbia. He was left out the matchday squad for the game against Italy, but was an unused substitute in the match verses Serbia.
He made his debut for the team in August 2012 as a substitute, in a 2-0 loss to Iceland in Reykavik.