[citation needed] Gasnier played his junior rugby league in the St George District with the Mortdale based Renown United club.

[3] Gasnier made his first Grade début for the St. George Illawarra Dragons in round 8 of the 2000 season against the Newcastle Knights.

Whilst in origin camp on a drunken bonding session with teammates, Gasnier left a lewd voice message on a woman’s phone.

In early 2007 it was announced that Mark Gasnier, Ben Hornby and Jason Ryles would captain St George Illawarra.

[citation needed] However, Gasnier tore his pectoral muscle tackling Nigel Vagana in the Charity Shield pre-season trial and was unable to play again until July.

On 21 July, Gasnier made his long-awaited return to the Dragons, in the five-eighth position, with his team winning 38–20 against the Penrith Panthers.

In July, Gasnier reported that he would switch to rugby union at the end of the season to link up with French club Stade Français.

[citation needed] Gasnier joined French rugby union club Stade Français from October 2008 on a two-year deal worth approximately €300,000 a season.

[8] Gasnier continued his prolific 2009–10 season by scoring his 6th try of the campaign against Bayonne in round 15 in front of more than 70,000 fans at the Stade de France in Paris.

He went on to play for the Dragons' in the 2010 NRL Grand Final, scoring the first try of the match to help the joint-venture club beat the Sydney Roosters and claim its first premiership.

[12][13] He finished his career in a semi-final clash at Suncorp Stadium against the Brisbane Broncos, with St. George Illawarra losing 13–12, after a Darren Lockyer field goal in extra time.

[15] On 20 July 2022, Gasnier was named in the St. George Dragons District Rugby League Clubs team of the century at centre.