Lagum was born to her single mother Sidonia Ayaa in Kampala. After losing her job, her mother relocated to Kitgum to move in with her grandparents. In her early years, Lagum was teased due to being much taller and thinner than her peers. As a child, she aspired to become a lawyer. Lagum moved back to Kampala to continue her schooling in St. Jude Primary School, Katikamu Seventh Day Adventist Senior Secondary School and St. Lawrence High School.
During her teenage years, several friends suggested that with her height and body she could work as a model. At the age of sixteen, Lagum began to pursue modeling as a career. Her first job was in Ugandan Fashion Week, where she walked for Gloria Wavamunno’s GW Collection. In 2012, Lagum met designer Adele Dejak, who took her to Kenya for a photo shoot.
In 2013, Lagum took part in a casting call for Africa's Next Top Model taking a 16-hour bus ride from Kampala to Nairobi. She was later called by host Oluchi Onweagba and was chosen to fly to Cape Town as one of the twelve finalists for the show.
Lagum quickly gained notice from the judges and guests of the show, winning several best photos along with the opportunity to appear in an editorial spread for South African Elle for having received best picture in week 9. Lagum made it to the finale on the tenth week of the competition, having made no appearances in the bottom two. After a casting session with DNA Models, she was crowned as the winner of the show. On her win, host Oluchi Onweagba explained, "We just felt like Aamito's look is so modern right now. I think she's a true representation of an African beauty, and she has a warrior spirit inside of her. I'm going to go to bed at night tonight and sleep so well because I know she can pull this off."
As part of her prizes Lagum received a 1-year modeling contract with DNA Model Management, product endorsement deals with P&G, Etisalat, Snapp and Verve International, a 1-year contract as an ambassador for South African Tourism and a cash prize of $50,000 USD.
After winning Africa's Next Top Model, Lagum was signed with Boss Model Management in South Africa and was featured in an editorial for Marie Claire South Africa in August 2014. She later signed with DNA Model Management and relocated to New York City. Lagum debuted in the F/W 2015 season, walking for the likes of Lacoste, Marc by Marc Jacobs, J. Mendel, Rag & Bone, Giles, Jonathan Saunders, Paul Smith, and Bottega Veneta. She then went on to open Balenciaga in Paris, making her the first model of colour to open the show under Alexander Wang. She continued her streak, booking more shows like Lanvin, Dries van Noten, H&M, Giambattista Valli, and Hermès.
At the end of the season, she was listed as one of the Top Newcomers by models.com and style.com. Since then, she has appeared in various magazines, including i-D, Interview, Love, W, V, American Harper's Bazaar, Italian and South African Elle, L'Officiel, Italian, Ukrainian, British, German, Mexican, and Spanish Vogue, Teen Vogue, and Italian Vanity Fair. In April 2017, she was featured on the cover of American Allure magazine. She got a cover of British Elle for August 2017. Lagum has also appeared in Lookbooks for Hugo Boss, Area, Narciso Rodriguez, Kenzo, Acne Studios, Guerlain, Nordstrom, Barneys New York, Sonia Rykiel, Saks Fifth Avenue, as well as advertorial for Kenzo and an advertising campaign for Marc Jacobs, shot by David Sims.
In October 2015, Lagum was ranked by Cosmopolitan as one of the most successful contestants of the Top Model franchise, alongside fellow winners Ksenia Kahnovich (Ty — supermodel, 2004), Alice Burdeu (Australia's Next Top Model, 2007), Montana Cox (Australia's Next Top Model, 2011), Sora Choi (Korea's Next Top Model, 2012), Hyun Ji Shin (Korea's Next Top Model, 2013), Oliver Stummvoll (Austria's Next Topmodel, 2014), and other alumni Fatima Siad (America's Next Top Model, 2008), Patricia van der Vliet (Holland's Next Top Model, 2008), Saara Sihvonen (Suomen huippumalli haussa, 2010), Leila Goldkuhl (America's Next Top Model, 2012) and Kate Grigorieva (Top Model po-russki, 2012).