So much of Junior Poluleuligaga's career depended on his great rival Steve So'oialo. Polu began his career by breaking into the North Harbour provinacal side, he was spotted for the Samoan national team and in 2006 was selected to nationalise to play for his parents country, representing composite side Pacific Islanders as a Samoan, having been unwanted for the 2004 tour due to the presence of So'oialo.
Following the success of the Pacific Islanders tour Polu represented Samoa in the IRB Pacific Nations Cup in the summer of 2007 due to the absence of So'oialo. Here he was again 2nd choice to So'oialo, although his luck was about the change.
So'oialo and Polu were both selected for the 2007 RWC squad, as Samoa looked to upset 2003 champions England and eventual champions South Africa. Polu was selected ahead of So'oialo for the opening game against the springboks, which they lost despite an encouraging start. So'oialo was restored for the derby game against Tonga which despite entering as overwhelming favourates lost 19–12. Polu was once again restored for the do or die game with England setting up an excellent try for Mahonri Schwalger. Having travelled north to France for the world cup Polu joined RC Toulon on a short term deal.
In the off season before the 2008/09 season he signed for Harlequins ironically moving to join So'oialo, and has since ousted his great rival for the bench spot behind England scrum half Danny Care. When his Harlequins contract finished in the summer of 2009 he joined the Bay of Plenty Steamers in the Air New Zealand Cup.
On 17 August 2010 Exeter Chiefs announced that they had completed a deal and visa for Junior. It was announced in local media in Devon, England that Polu will be leaving Exeter Chiefs at the end of the 2012/13 season.