Sosene joined the Labour Party in 2000.[2] She was first elected to the Māngere-Ōtāhuhu local board in the 2010 local elections. She was re-elected in the 2013 local elections and 2016 local elections. The board elected her chair in 2013 and returned her to that position in 2016 and 2019.[2][3][4] In her role as chair of the board, Sosene has spoken about the impact of overcrowded housing on Pasifika, and the benefit to Samoans in Auckland of a rise in the minimum wage.[5][6] She supported the extension of Auckland light rail through the Māngere town centre.[7][8] She resigned from the board in May 2022, following her election to Parliament.[9]
At the 2017 general election, Sosene was a list-only candidate placed 44th on the Labour Party list.[10] Labour did not win sufficient representation for Sosene to be elected.
At the 2020 general election, Sosene was again a list only candidate for the Labour Party, ranked 54th.[11] Although Labour won more than 63 seats, the election of twelve lower-ranked or unranked constituency candidates prevented Sosene's election at that time; however, she was still invited to participate in the new MP induction process while waiting for the special votes to be counted.[12] She was the highest-ranked Labour list candidate who was not elected at the general election, but she was sworn in as a Member of Parliament on 2 May 2022, following the resignation of list MP Louisa Wall.[13][14]
In 2023, Sosene won the Labour nomination for the safe Labour seat of Māngere, succeeding Aupito William Sio who retired.[15] On 14 October, she won the seat by a margin of 11,712 votes over National's Rosemary Bourke.[16]
On 30 November 2023, Sosene was appointed as spokesperson for Internal Affairs, associate Pacific Peoples, and associate Social Development and Employment in the Shadow Cabinet of Chris Hipkins.[17]