After failing to earn selection into the prestigious Clairefontaine academy, Jullien joined another local Parisian club, US Torcy, the same club that produced France national team's Paul Pogba. He then left in 2005 for Auxerre. Jullien spent six years in the youth and senior teams at the club, before being spotted by SC Freiburg.
Auxerre
After having spent his first two years in football, Jullien signed up on an aspirant contract with first division club Auxerre at age 12. He quickly established himself in the club's youth system. After a year spent playing with the club's reserve team, former coach Jean-Guy Wallemme started playing him during the 2012–13 campaign in the second division. He made his club debut on 24 August 2012, when Auxerre won 2–1 against Stade Lavallois. Jullien scored his first professional goal a week later, in a League Cup match against Dijon.
SC Freiburg
Jullien signed his first professional contract with SC Freiburg on a free transfer, after Auxerre failed to do so in the summer of 2013. During his first season in Freiburg, although he only was capped for the second team, he played in the fourth tier, the Regionalliga.
Dijon (loan)
In late June 2015, Jullien was loaned to Dijon FCO in the Ligue 2.
On 1 July 2016, Jullien signed a four-year contract for Ligue 1 side Toulouse.
Celtic
On 28 June 2019, Jullien signed a four-year contract for Celtic. He signed for a fee rumored to be in the region of £7 million, which would make him Celtic's second most expensive signing. Jullien was given the number 2 shirt, vacated by the departure of Jeremy Toljan. In the following game against Romanians CFR Cluj, Jullien was left on the bench as Celtic crashed out losing 4–3 on the night and 5–4 on aggregate in a Champions League Qualifying 3rd round 2nd leg tie for the 2nd successive season, the manager was heavily criticised for his team selection the following day, laying the blame on him for conceding four goals. He scored his first goal for Celtic in a UEFA Europa League qualifying tie against AIK on 29 August 2019. He suffered a head injury in a gamein September 2019. Jullien won his first domestic trophy, scoring the only goal, in a 1–0 victory over Rangers in the 2019 Scottish League Cup Final on 8 December at Hampden Park.
