In 1977, Duffy landed the role of Mark Harris in the short-lived television series "Man from Atlantis". Following the series' cancellation in early 1978, he got his big break in the role of Bobby Ewing, opposite Barbara Bel Geddes and Larry Hagman, on the prime-time soap opera "Dallas". The show became a worldwide success. Despite its success, Duffy opted to leave the series in 1985 with his character being killed off onscreen. However, with both the show and his career on the decline, he returned in 1986 in the infamous shower scene that rendered the entire 1985–1986 season "just a dream." Duffy then remained with the series until its cancellation in 1991. He also appeared in several episodes of the spin-off series "Knots Landing" between 1979–82. Throughout the 13-year run of "Dallas", Duffy directed several episodes of the series. Along with "Dallas" fame, Duffy has also tried his hand at singing, and in 1983, he had a hit in Europe with "Together We're Strong", a duet with French female singer Mireille Mathieu. The single reached #5 in the Netherlands in April 1983.