Marmion scored his first try for Connacht on 28 September 2013, against Ospreys in the 2013–14 Pro12. Marmion again featured in all of Connacht's competitive games for the season, playing in the league and the 2013–14 Heineken Cup, with Pro12 matches against Benetton Treviso and Munster the only two games that he did not start in. Marmion signed a further contract extension with Connacht in November 2013, which will see him remain with the province until at least the summer of 2016.