Her mother, Pauline Belhumeur, left her job as a schoolteacher in 2013 to act as a talent manager for both Sophie and her younger sister Isabelle, who is also an actress.

[8][9] Additionally, Nélisse formed and still maintains a close friendship with her co-star from The Book Thief, Nico Liersch.

[11][12][13] In 2016, Nélisse was one of four actors selected by the Toronto International Film Festival, alongside Grace Glowicki, Jared Abrahamson, and Mylène Mackay for its "Rising Stars" programme, described as an "intensive professional development programme [that] immerses participants in a series of public events and industry meetings during the Festival.

[15] After graduating high school in June 2017,[16] Nélisse confirmed she wouldn't be enrolling in post-secondary education for the time being due to commitments shooting Close in London and Morocco.

[18] One of these instances was when she turned down the sexually-charged lead role in Fugueuse, in which Ludivine Reding was eventually cast as Fanny, the titular teenage runaway who ends up a sex trafficking victim.