Krajnc began his football career at the age of eight, when his father took him to the Slovenian club Maribor for a trial.

[2] He signed his first professional contract with Maribor at the age of 16 and played for the main squad in a couple of friendly matches.

[8] With 18 years, one month and 14 days, he became the youngest Slovenian player with an appearance in the Italian top division, Serie A.

[18] He made his senior international debut on 30 March 2015, replacing Dominic Maroh after 65 minutes in an eventual 1–0 friendly defeat against Qatar at the Jassim bin Hamad Stadium in Doha.

[19][20] Krajnc is from Kapla na Kozjaku in the hilly region of northeastern Slovenia near the Austrian border, approximately 37 km from the city of Maribor.