In the late 40s, Forsythe began working in television, the medium that would occupy most of his career. He appeared in many of the New York-based live TV series such as "Studio One", "Lights Out", "Schlitz Playhouse of Stars" and "Climax". Alfred Hitchcock, who cast Forsythe in the 1955 black comedy, "The Trouble with Harry" and in several episodes of "Alfred Hitchcock Presents", reportedly advised the handsome leading man that he was best suited to television work. This was good advice (even if Hitchcock himself would later cast him in "Topaz", his 1969 political thriller feature).

Forsythe made his mark in debonair paternal parts in several long running television series. In "Bachelor Father" (1957-62), he was a playboy Hollywood attorney who was responsible for raising his orphaned niece. "To Rome with Love" (1969-71) found him portraying a widowed American college professor raising three daughters in Rome. He also provided the avuncular voice for the perpetually off-screen Charlie Townsend in "Charlie's Angels" (1976-81).

"Dynasty" (1981-89), the immensely popular primetime soap opera, afforded the middle-aged Forsythe a change of pace characterization and the most successful role of his career. As Blake Carrington, the steely patriarch of the wealthy, tempestuous Carrington clan of Denver, Colorado, he became a TV matinee idol. Whether he was wheeling and dealing in his business affairs or glowering at his treacherous ex-wife, Alexis (Joan Collins), Forsythe displayed elegance and style; his Blake Carrington became a significant icon of the Reagan era; in the words of the "Entertainment Celebrity Register", he was "capitalism's most attractive symbol." In 1985, Forsythe even became the spokesperson for a men's cologne line called "Carrington".