When he was sixteen years old, he was spotted at the New York Boys' Club on East Tenth Street by Pavel Tchelitchev, who recommended him to Lincoln Kirstein as a scholarship student at the fledgling School of American Ballet.

He first appeared on stage in American Ballet Caravan's production of A Thousand Times Neigh, a tribute to the automobile, at the Ford pavilion at the 1939 New York World's Fair.

[9] During this time he collaborated with Balanchine to create several roles including: The Poet in La sonnambula (The Night Shadow), Cleónete in Le Bourgeois Gentilhomme and Jean de Brienne in Raymonda.

[23] In later years he was also featured on several other broadcasts including: Camera Three for CBS as Prince Siegfried in Swan Lake (1956), Omnibus in A Midwinter Night's Dream (1961) and episodes of The Bell Telephone Hour for NBC (1962–1964).

[24] His performances on film included a collaboration with Tanaquil Le Clercq dancing La Valse (1951) and with Louis Falco in a production of Dionysus in the role of Pentheus (1963).

[25][26] In 1967 he also collaborated with Suzanne Farrell, Edward Villella and Francisco Moncion in Balanchine's A Midsummer Night's Dream dancing the role of Lysander.

With no understudy on standby, Magallanes learned the technically challenging part in one day's rehearsal and danced the Cavalier of the Sugar Plum Fairy.

Depicting a gathering of Russian éemigré artists in the 1940s, the play includes a scene in which Balanchine choreographs Orpheus on Magallanes and Tallchief as Stravinsky looks on.