The second of seven children[1] of Hjálmtýr E. Hjálmtýsson, a bank clerk, and Margrét Matthíasdóttir, a writer, Diddú was born on 8 August 1955[2] and raised in Reykjavík.
She was a guest singer as Susanna in The Marriage of Figaro in Trondheim, Norway, and as Gilda in Rigoletto in Gothenburg, Sweden (1992).
She performed "Il Bacio" ("The Kiss"), "Mein Herr Marquis" from Johann Strauss II's Die Fledermaus, the cavatina "Casta diva" from Bellini's Norma, and "É strano... sempre libera" from Verdi's La traviata; and sang in duet with Garðar Thór Cortes in "O soave fanciulla" from Puccini's La bohème, in a duet from act 1, scene 5, of Verdi's Rigoletto, and in "The Prayer" by Carole Bayer Sager.
[2][8] Her father, Hjálmtýr E. Hjálmtýsson (5 July 1933 – 12 September 2002),[9] had roles in the Icelandic comedies Með allt á hreinu (On Top, 1982),[10] Löggulíf (A Policeman's Life, 1985)[11] and Karlakórinn Hekla (The Men's Choir, 1992).
[12] Diddú's youngest brother, Páll Óskar Hjálmtýsson, is a pop singer, songwriter and disc jockey.