Koscina was born to parents from Split and Trogir ("Košćina" in čakavian Croatian means "a great bone"; Košćina was a family name of the owner of the first greater shipyard in Split, established 1831). Her publicity agents invented that she was born "Sylva Koskinon" of a Greek father and a Polish mother. She may be best-remembered for her role as Iole, the bride of Hercules (Steve Reeves) in Hercules (1958) and Hercules Unchained (1960). She also played Paul Newman's romantic interest in The Secret War of Harry Frigg (1968).
During the Second World War when she was a teenager, she moved to Italy to live with her sister, who had married an Italian citizen.
Koscina had an extensive film career there. She also starred in the 1967 comedy caper Three Bites of the Apple with David McCallum, and Deadlier Than the Male (1967), in which she and Elke Sommer portrayed sophisticated professional killers dueling with Bulldog Drummond. She also played Danica in the Yugoslavian movie The Battle of Neretva, in 1969. She played a German doctor, Bianca, in Hornets' Nest with Rock Hudson.
Koscina had studied physics at the University of Naples, was Miss Di Tappa at the Tour of Italy bicycle race in 1954, as well as being a fashion model. She made a fleeting appearance in the part of an aspiring actress in Siamo uomini o caporali? [Are we men or corporals?] (1955) before making a flying catch at her great opportunity: she portrayed Giulia, daughter of the train engineer Andrea, in Pietro Germi's Il ferroviere [The Railroad Man] (1956). Koscina immediately confirmed her talent in Guendalina (1957), where she had no difficulty playing the part of a young mother.
A lead player in popular comedies, such as Nonna Sabella [Grandmother Sabella] (1957), Ladro lui, ladra lei [He a thief, she a thief] (1958), and Poveri millionari [Poor millionaires] (1958), Koscina alternated cleverly between roles as vamp and ingenue. She represented women in search of social upward mobility, the image of an Italy that had left its worst problems behind.
Koscina was suited to sophisticated comedies like Mogli pericolose [Dangerous wives] (1958), where she made a direct sentimental challenge to poor Giorgia Moll. But she also seemed at ease draped in a peplum: she made a marvelous fiancée for Hercules in Le fatiche di Ercole [Hercules] (1958), a prototype of this kind of film. A true-life example of her popularity in Italy occurred when, in order to win her over, a police officer let her go without issuing a traffic ticket. Sylva was later a guest on a television program, at which time she thanked the policeman on the air, thus getting him into lots of trouble with the police department. The incident and its aftermath inspired the movie Il vigile [The policeman] (1960), where she played herself.
In the first half of the sixties, Koscina married her lover, Raimondo Castelli, a small producer connected with Minerva Films. She managed to keep well afloat with roles in Damiano Damiani's Il sicario [The hired killer] (1961). In La lepre e la tartaruga [The Tortoise and the Hare], an episode in Le quattro verita [The Three Fables of Love] (1963), the director Blasetti constructed a duel between Koscina and Monica Vitti. In 1965, Koscina appeared in Giulietta degli spiriti. She was also a television personality, as she was often the special guest on variety shows.
From the early 1960s, she invested most of her considerable salaries as a star in a luxurious villa, in the well-to-do district of Marino, Rome, complete with 16th-century furniture and artistic paintings. That lasted until her spending overcame her dwindling income, and she had to face a tax evasion inquest, when she was forced to sell her house in 1976. Living with Raimondo Castelli since 1960, they did not marry due to then Italian law and because his wife Marinella refused him an annulment. Castelli and Koscina married in Mexico in 1967, but that marriage was not recognized in Italy.
After passing thirty, she partnered with actors such as Kirk Douglas in A Lovely Way to Die (1968) and Paul Newman in The Secret War of Harry Frigg (1967), but without any luck. Her fame being a bit tarnished, it was given a boost in the second half of the sixties when she was photographed bare-breasted in the Italian edition of Playboy magazine. Mauro Bolognini's L'assolute naturale (1969) was released complete with a "chaste" full nude shot.
- On her infamous love scene – "Of course, if it had not been for the director, I wouldn't have done this film."