[3] Mikolaj's artistic career began at the Teatr Wielki in Poznań (1995–2002), where she performed as Countess Almaviva in Mozart's Le nozze di Figaro, Pamina in The Magic Flute and Rachel in Meyerbeer's La Juive.

[3] She was a member of the ensemble of the Bavarian State Opera in Munich from 2002 to 2007,[2][7] appearing as Pamina, Countess Almaviva, Donna Elvira in Mozart's Don Giovanni, Ännchen in Weber's Der Freischütz, Drusilla in Monteverdi's L'incoronazione di Poppea, Gretel in Humperdinck's Hänsel und Gretel, Marzelline in Beethoven's Fidelio, Euridice in Gluck's Orfeo ed Euridice, Najade in Ariadne auf Naxos by Richard Strauss, Almirena in Handel's Rinaldo, and Musetta in Puccini's La Bohème.

[2][7] Mikolaj appeared as a guest at European opera houses, performing roles such as Pamina and Eva in Wagner's Die Meistersinger von Nürnberg at the Aalto-Theater, Countess Almaviva at the Opéra Bastille and Micaela in Bizet's Carmen at the Vienna Volksoper.

[8] In the 2008/09 season, Mikolaj appeared as Donna Elvira at the Opéra de Monte-Carlo, at the Hessisches Staatstheater Wiesbaden, the New National Theatre Tokyo and at the Vienna State Opera and guest performance at the Budapest Spring Festival.

[6] She appeared in a joint production of the Berlin State Opera and La Scala in Milan, as Woglinde in Wagner's Der Ring des Nibelungen conducted by Daniel Barenboim, which was also presented at the Proms.

[6] A reviewer of a 2015 concert with the Boston Philharmonic Orchestra conducted by Benjamin Zander noted that, in Four Last Songs by Richard Strauss, "she exhibited a degree of abandon and rapture" and her "sublime rendering of 'So tief in Abendrot'".