He made his film debut in 1937 in Daphne and the Diplomat, followed by German movies, such as Heimat (with Zarah Leander), Friedrich Schiller – The Triumph of a Genius (with Horst Caspar) and The Great King (with Otto Gebühr).
After World War II, Nielsen performed in a Kabarett group, Die Außenseiter ("The Outsider") and played in revues by cabaret artist Günter Neumann.
He also worked with Curd Jürgens and Lilo Pulver in a film version of Conrad Ferdinand Meyer's Gustav Adolf's Page.
He also appeared in a few Karl May films and in Edgar Wallace's The Indian Scarf, Das Phantom von Soho and The Door with Seven Locks.
In addition to his own film roles, in 1948, Nielsen began working as a voice actor, dubbing films into German, including Fred Astaire (Funny Face and Daddy Long Legs); Gary Cooper (Cloak and Dagger, among others); Errol Flynn (The Adventures of Robin Hood, Adventures of Don Juan, among others); Cary Grant (Crisis); Rex Harrison (Cleopatra); Phil Silvers (Cover Girl); David Niven (The King's Thief, among others), Tyrone Power (The Mark of Zorro and Prince of Foxes, among others); James Stewart (in The Philadelphia Story and Anatomy of a Murder, among others); Robert Taylor (Quo Vadis), Spencer Tracy (Malaya); Orson Welles (Citizen Kane) and Trevor Howard (The Third Man).