Ihnat was born in Slovakia and was raised on a farm in Lynden, Ontario. His family settled there after fleeing his native Czechoslovakia in 1939, when he was five years old. Ihnat, his mother, father, younger sister, and two young boys from other families left Czechoslovakia three days before Prague was occupied by invading German forces in March of that year.

Ihnat moved to the United States in 1958 to pursue a career in acting and attended the Pasadena Playhouse. He gained United States citizenship. At a time when he had difficulty finding work he enlisted in the U.S. Army for two years and served at Headquarters and Headquarters U.S. Army, Port Inchon, South Korea. In 1960, Pvt. Ihnat won second prize in the Republic of Korea poetry contest for his entry titled "Toil in the Night."

Ihnat guest-starred in many television series during the 1960s. including a mind-controlled lieutenant in the science fiction television series The Outer Limits in the two-part episode, "The Inheritors", (1964). In 1965 he guest starred as murderer Charlie Parks in the Perry Mason episode, "The Case of the Duplicate Case." In 1959 he played a truck driver in with cargo hijackers in an episode of "Highway Patrol".

Ihnat was married to Marya Carter, who posed as Playboy's Playmate of the Month for May 1962. Carter had a daughter from a previous marriage.

From 1964 to 1968 he appeared in eight feature films. He often played villains, using his abilities to subtly turn one-dimensional characters into complex and multi-dimensional antagonists. In 1968, Lamont Johnson cast him in the film Kona Coast in which Ihnat played a murderous playboy in Hawaii doping up teenagers and causing mayhem to the property and person of the character played by lead actor Richard Boone. Also in 1968, he memorably portrayed a murderous thug in the film Madigan, starring Richard Widmark and Henry Fonda and a NASA administrator in the film Countdown, directed by Robert Altman and starring James Caan and Robert Duvall. His other film credits included The Chase (1966), In Like Flint (1967), Hour of the Gun (1967), Zig Zag (1970), and Fuzz (1972).

Ihnat had several guest roles in Mission: Impossible including the brilliant Soviet Union investigator Stefan Miklos in the 1969 episode "The Mind of Stefan Miklos," widely praised as one of the most cerebral and intelligent episodes of the entire series. While he played other roles (mostly villains, like in "The Astrologer") in the show, his performance in this episode is his most memorable.

Ihnat held over seventy guest credits in such well known series as Star Trek episode "Whom Gods Destroy" (1969) as the psychotic Garth of Izar and also two episodes of The Fugitive, entitled "Cry Uncle" (alongside Ron Howard) and "The Walls of Night". Other credits include Blue Light ("Field of Dishonor"), Gunsmoke ("Exodus 21:22", "Jenny" with Lisa Gerritsen (December 28, 1970), and "Noose of Gold"), The Silent Force ("Take As Directed For Death"), Bonanza ("A Dream to Dream" and "Terror at 2:00"), The Virginian ("Jed" and "Last Grave at Socorro Creek"), Mission: Impossible ("The Astrologer"), ("The Mind of Stefan Miklos"), Cimarron Strip ("The Hunted"), I Dream of Jeannie ("My Master the Rainmaker"), Mannix ("Huntdown", "End Game" and "To Draw the Lightning"), The F.B.I. ("Region of Peril" and "The Prey"), The Name of the Game ("The Chains of Command" and "Nightmare"), Medical Center ("Fright and Flight") and Perry Mason ("The Case of the Duplicate Case").