He has played mostly military roles in films such as The Last King of Scotland (Masanga), Sometimes in April (Colonel Bagosora), The Silent Army (Michel Obeke) and The Mercy of the Jungle (Major).
Nkaaga started out as a supporting cast member at his uncle Omugave Ndugwa's theatrical drama group The Black Pearls in 1988.
When Kato Lubwama and some of his friends left The Black Pearls in 1993, Nkaaga also started Afri Talent with his other colleagues Ruth Wanyana, Brenda Nanyonjo, Michael Mabira, Bob Bulime and John Segawa.
His breakout role in Afri Talent was when he acted as Chombe, a ruthless leader who didn’t care about the plight of soldiers' families.
His other works in theatre include Ekitangaala Munzikiza, Omuyaga Mu Makoola, Omusaayi Gw’obutiko, Olujegere Lw’obulamu, Galimpitawa with The Black Pearls and Order, The Best of Abby and Patricko, Ensitaano, Safari, Sebalamu, Tebesigwa, Mbyaase and Akandolindoli at Afri Talent.