Hemmings was born on 16 July 1996, and raised in the Freemans Reach suburb of the Hawkesbury district in New South Wales. His father—Andrew Hemmings—was previously employed as a maintenance worker, while his mother—Liz Hemmings—is a former accountant turned high school mathematics teacher, now working in photography. Hemmings has two older brothers: Ben and Jack, both who previously worked as builders, with the latter now owning a street-wear clothing label. Hemmings has spoken on his working-class upbringing; elaborating: “[The band’s] beginnings were so humble. […] I grew up in a small house in the middle of nowhere”. When he was 10 years old, Hemmings’ brothers taught him how to play guitar, later prompting him to take professional lessons and learning from Youtube tutorial videos to learn how to play the instrument. Eventually, Hemmings began busking throughout his tween years.

In 2011, at the age of 14, Hemmings began posting YouTube song cover videos, under the username “hemmo1996”. Hemmings’ first video, a cover of Mike Posner’s “Please Don’t Go”, was posted on 3 February 2011. As Hemmings’ covers began gaining traction on the platform, he invited Hood and Clifford to join his videos. The trio eventually added mutual friend Ashton Irwin to their videos, forming the current 5 Seconds of Summer lineup. After months of posting song covers together, the band began attracting interest from major music labels and publishers and signed a publishing deal with Sony/ATV Music Publishing. Hemmings has since released four studio albums with the band, each met with worldwide success: 5 Seconds of Summer (2014), Sounds Good Feels Good (2015), Youngblood (2018), and Calm (2020).

Following the band’s rise to fame, Hemmings continued with his high school education, opting to switch to distance learning. In 2013, he chose to not finish Year 12, the final year of secondary schooling in Australia, due to his commitment to the band.

In 2015, Hemmings began an on-again off-again relationship with social media influencer, Arzaylea Rodriguez, after they met at Kylie Jenner’s birthday party. The couple ended their relationship in May 2017. In August 2018, it was confirmed that Hemmings was dating singer Sierra Deaton. The couple currently reside together in Los Angeles.

Apart from the band, Hemmings has also pursued modelling. Hemmings, along with his band-mates, is currently signed to the celebrity division of Wilhelmina Models. In February 2019, Hemmings attended New York Fashion Week, debuting as a model and walking the runway for Philipp Plein’s Autumn/Winter 2019 Ready-To-Wear collection. In July 2019, Hemmings starred in Numéro Homme’s video editorial. In December 2019, Hemmings was featured as the cover model on the Winter 2019 issue of Glass Man magazine.

As of 2020, Hemmings’ net-worth is estimated to be $25 million (USD).