A central defender, Jeanvier began his career in 2000 with hometown club US Clichy-sur-Seine in France.

He moved to L'Entente SSG and Clairefontaine before joining the youth setup at Nancy in 2008. He progressed to the reserve team Nancy II and made 54 appearances for them in the Championnat de France Amateur, scoring twice.

When Patrick Gabriel, former Head of Youth Development at Nancy, was promoted to First Team Coach in 2013, Julian earned a spot in the senior side. He played 10 times but would move to Lille the following season.

He signed for the Ligue 1 club in July 2013, signing a four-year deal. A surplus of defenders lead to loan spells at Belgian side Mouscron-Péruwelz for the 2014-15 season and then Red Star in Paris for 2015-16.

He left permanently in August 2016, joining Ligue 2 side Reims. He became a mainstay in the defence and was named in the UNFP Ligue 2 Team of the Year in consecutive seasons, 2016-17 and 2017-18. The summer section of the 2018 season saw him vice-captain Reims on their way to promotion to Ligue 1 as champions of the second tier.

In July that year, he signed for Brentford in the Sky Bet Championship. He scored a goal in each of his first two games for the club, which came against Southend United and Cheltenham Town in the EFL Cup.

A foot injury ruled him out for two months that season but the next year would see him play 27 times behind Pontus Jansson and Ethan Pinnock.

A loan move to Kasimpaşa in Turkey gave him a starting berth but a season-ending cruciate injury cut short his spell away from West London. It also ruled him out of contention for the successful Play-Off campaign that saw Brentford promoted to the Premier League in May 2021.