Lonsdale was born in Sydney, Australia, to a Nigerian father of the Edo people, and an Australian mother who is of Irish and Danish descent. He is the youngest of six children on his mother’s side, with three brothers and two sisters, and one of seven from his father’s side; in total he has 11 siblings.
Lonsdale got his first acting job in 2007, with a bit dancing part in the film Razzle Dazzle: A Journey into Dance. The following year, he appeared in an episode of the Australian television medical drama All Saints.
Lonsdale became a recurring member of the cast of the Australian teen drama series Dance Academy in the 2012 second season; his character Oliver Lloyd became a regular in the third and final season.
He was a VJ on MTV for two and a half years. He released a single on iTunes titled “One and Only” in 2014. He then played a supporting role as Uriah Pedrad in The Divergent Series: Insurgent (2015), the second film in the series. He briefly reprised the role in The Divergent Series: Allegiant (2016). In 2016, his single “Higher” was included on Connor Franta’s curated album Common Culture, volume He next appeared in the historical drama The Finest Hours (2016) as Eldon Hanon, the youngest sailor involved in a rescue at sea.
In 2015, he auditioned for the role of Jefferson “Jax” Jackson—a recurring character introduced in the second season of the CW’s The Flash—but this role was given to Franz Drameh. Instead, Lonsdale was cast as Wally West, the previously unknown son of the hero’s foster father, who became a regular supporting character as Kid Flash. In 2017, his character left The Flash and debuted on Legends of Tomorrow. That year, Lonsdale also appeared in an episode of Supergirl.
In 2018, Lonsdale starred as Bram in the film Love, Simon, based on the novel Simon vs. the Homo Sapiens Agenda, by Becky Albertalli, about a teenage boy who struggles to come out; Lonsdale has said that the role helped him come to terms with his own sexuality. Upon release, the film received positive reviews from critics, and was dubbed historically significant, as it is the first film ever released by a major studio to focus on a gay teenage romance.
Lonsdale has also released musical recordings, including “Higher” in 2015 and “Good Life” in 2017 and “Kiss The Boy” in 2018 and “Preach” in 2018.
Lonsdale came out as being attracted to people regardless of gender on social media in 2017. Although many news outlets initially reported that he was pansexual, Lonsdale prefers not to label his sexuality.