Marius had an extensive career acting in television productions over nearly fifty years from the 1930s right through to 1980s.
His earliest television work was in two short films, both for the BBC. The first was The Bear in 1938, a one-act comedic play written by Anton Chekhov, co-starring his future wife, Lucie Mannheim, in their first filmed appearance together. The second was Box for One in 1949, written by Peter Brook, who would later become famous for his work with the Royal Shakespeare Company and is considered to be Britain's greatest living theatre director. All these productions are now considered to be lost.
After the war and his work in some of his most famous films, he started to appear more frequently in television productions from the early 1950s, including an appearance in Douglas Fairbanks Jr Presents (NBC, 1953-57): 'The Rehearsal' (1954) and two plays for the Lilli Palmer Theatre (NBC, 1955-56): 'Mossbach Collection' (1955) and 'Episode in Paris (1956)'.
His first major television series was The Adventures of the Scarlet Pimpernel (ITV, 1955) where he starred as Sir Percy Blakeney aka The Scarlet Pimpernel. It was a rôle which he had also performed in two different radio shows: a six episode series for the BBC in 1949 and a 52 episode series in 1952-53 for US radio (NBC) which was later broadcast on the BBC. He co-produced the eighteen episode TV series and wrote one episode ('The Flower Woman') that prominently featured his wife, Lucie Mannheim, who co-starred with him in seven episodes of the series.
In the late 1950s, he performed in two German television productions of English plays. The first was Gaslicht (Gaslight) (Süddeutscher Rundfunk (SDR) 1956) by Patrick Hamilton and directed by the prolific Austrian director, Rudolph Cartier. Marius played the leading rôle of Jack Manningham, the name of the character in the original stage play, who was the duplicitous and murderous husband. It was a rôle also famously performed (under a different character name) by Anton Walbrook (his co-star in The Red Shoes (1948) in the 1940 British film 'Gaslight' and by Charles Boyer in the 1944 Hollywood version with Ingrid Bergman. Marius performed it in German, a language in which he was fluent.
He later played another rôle in German: Lord Goring in Ein Idealer Gatte (An Ideal Husband) (Nord und Westdeutscher Rundfunkverband (NWRV) (1958). This was a production of the renowned Oscar Wilde play. One of his co-stars (as Sir Robert Chiltern) in the production was German actor, Albert Leiven, with whom Marius performed several times throughout his screen career. Marius later noted in an interview that while Germans really appreciated Oscar Wilde, his original English dialogue did not always translate entirely successfully into German.
Throughout the 1950s, he performed in nine plays in the BBC Sunday-Night Theatre (1950-57) series, playing fictional and real characters as diverse as Hjalmar Ekdal in Ibsen's 'The Wild Duck' (1952), Archbishop Cranmer in 'The White Falcon' (1956), Crystof Wolters in 'The Cold Light' (1956) and Robert Clive in 'Clive of India' (1956).
He played Theodore Maxtible in The Evil of the Daleks (BBC, 1967), a Doctor Who story. Over six of the seven episode story arc, he co-starred with Patrick Troughton (Doctor Who), who was his acting partner from The Adventures of the Scarlet Pimpernel. Patrick had played Sir Andrew Ffoulkes in most of the episodes of that earlier series. Unfortunately, only one of the seven episodes still exist in the BBC archive but fortunately that is an episode (Episode Two) that features Marius.
His longest running television rôle was as Doctor (later Professor) John Hardy, a forensic pathologist, in sixty two episodes of The Expert (BBC, 1968–1976). This production was groundbreaking in several ways: it was the first BBC2 series to be broadcast in colour and influenced subsequent US TV series featuring forensic pathologists such as 'Quincy M.E.' and 'CSI: Crime Scene Investigation'.
He also featured in Fall of Eagles in two episodes as General Paul von Hindenburg (BBC, 1974). This seven episode series followed the fortunes of three royal families, the Romanovs of Russia, the Hohenzollerns of Germany and Hapsburgs of Austria-Hungary in the years up to and including the First World War. As von Hindenburg, Marius played the ageing general, supreme commander of the Germany's armed forces during WWI, dealing with the capricious German Kaiser Wilhelm and the burdens of fighting a war that is being lost by his nation.
His next television rôle was once again as John Hardy in a fourth and last series of The Expert for the BBC in 1976. Five years on from the previous series, Hardy is now Professor of Forensic Medicine at Oxford and a consultant pathologist to the Inner London Coroner. He has just moved into a new house and lab with new staff, while still trying to get over the death of his wife five years before. The stories in these last ten episodes of The Expert present various dilemmas and difficulties for Hardy, with some testing his relationships with old friends and colleagues and even, at times, his belief in his own abilities.
He played the rôle of a real life person, King George V in the mini-series Edward & Mrs Simpson (Thames, 1979). He featured in the first two episodes as the ailing, then dying king, worried about the future of the monarchy when his eldest son, David (whom he considers unfit) takes over the throne after his death as King Edward VIII.
One of his last major television appearances was as Emile Englander in the five episode mini-series The Old Men at the Zoo (BBC, 1983) based on the 1961 dystopian novel by Angus Wilson.