The 19-year old right-back ended an injury-plagued stint with Heart of Midlothian earlier this summer to join Edinburgh rivals Hibs and it has already paid dividends as the club prepares to take on Swedish side Malmo in the second round of Europa League qualifying next week.
"It's good to have this game so early. Usually competitive games don't start until August but all the boys are pushing themselves really hard to get into the team," he said.
"It was a great experience just being able to sit and watch the games at Hearts and it was a great atmosphere. When you watch European games on the telly, you think that you'd love to play on that stage."
Of his move to Easter Road from Tynecastle, Mullen added: "When I was released there was a wait as I was still injured.
"I thought I'd have to drop a level but when the chance came to sign for Hibs I couldn't say no. I never expected it. I was shocked it was Hibs who called. I didn't have a clue they'd come in for me.
"But I'm not from Edinburgh so I'm not a Hibs or a Hearts supporter and that made it a bit easier.
"I think about it as doing a job."