His film career included roles in Madigan (1968), Ice Station Zebra (1968), Assault on the Wayne (1971), Look What's Happened to Rosemary's Baby (1976), The Greatest (1977), and Good Guys Wear Black (1978).

He played Communications Officer Alden in the second Star Trek pilot episode "Where No Man Has Gone Before" (1965), but was replaced by Nichelle Nichols as Lt. Uhura when the series went into production the following year.

[3] Haynes also appeared on television shows such as Hotel, The Green Hornet, The Fugitive, The FBI, Marcus Welby, M.D., and Emergency!, as Captain Stone of Los Angeles County Fire Station 8 in the fourth episode of its fifth season (1975-1976) called "Equipment".

He was best known as high school history teacher Pete Dixon in the comedy-drama series Room 222, with Denise Nicholas, Michael Constantine, and Karen Valentine.

During his illness, Haynes was co-starring in the television soap opera General Hospital as Mayor Ken Morgan and was commuting from Coronado to Hollywood for filming, as he was working up until the time of his death.