As a teenager, Johnson attended Girard College, a private boarding school for fatherless boys, located in Philadelphia.

[1] After graduating from high school, Johnson enlisted in the United States Army Air Forces as an aviation cadet.

[5] Johnson became a close friend of Audie Murphy and later appeared with him in three of his films, Column South and Tumbleweed in 1953 and Ride Clear of Diablo in 1954.

Johnson's Hollywood career began in 1952, with the college fraternity hazing exposé For Men Only, and with Loan Shark, also released in 1952 and starring George Raft.

In 1957 he played a ruthless and heartless murdering outlaw named "Stragger" on the TV Western Gunsmoke in the episode "Bloody Hands" (S2E21), where he almost convinces Matt Dillon to quit from guilt because of his own constant need to kill.

He appeared four times on the first-run syndicated military drama The Silent Service, based on actual stories of the submarine section of the United States Navy.

He was cast as Hugh Grafton in episode 28, "The Gar Story", as the executive officer Beach in the 1957 "Tirante Plays a Hunch", appeared twice as submarine officer and later author Edward L. Beach Jr., and as Tom Richards in two 1960 episodes, "Intermission" and "The Desperate Challenge", and twice with June Allyson on her CBS anthology series The DuPont Show with June Allyson.

Then he was cast as John T. Metcalf in the 1962 episode "Mile-Long Shot to Kill" of the CBS anthology series GE True, hosted by Jack Webb.

From January 1959 to May 1960, Johnson co-starred as Marshal Gib Scott on the television series Black Saddle, which lasted two seasons—its first on NBC and its second on ABC.

Johnson also appeared on The Outer Limits in 1964, playing a crewmember on a United States space station in the episode "Specimen: Unknown".

In a 2004 interview, Johnson commented about the role and shared his perspective regarding Gilligan's Island and the situation comedy's place in television history:It used to make me upset to be typecast as the Professor...But as the years have gone by, I've given in.

He appeared as a guest star in several dramatic series, including The Big Valley with Peter Breck (marking a reunion of sorts, since they co-starred together in Black Saddle, an earlier Four Star Productions series), The Invaders, Death Valley Days, Owen Marshall: Counselor at Law, Lassie, That Girl, Ironside, The F.B.I, Mannix[9] and Gunsmoke.

He was cast in the miniseries Vanished, based on a novel by Fletcher Knebel (1971), the TV horror movie The Horror at 37,000 Feet (1973), uncredited in the Robert Redford spy thriller Three Days of the Condor (1975), the low-budget thriller Hitch Hike to Hell (1977), and appeared on the episode "Coffee, Tea or Cyanide" on McMillan and Wife in 1977, and on the NBC soap opera Santa Barbara.

In an interview with Starlog magazine in the early 1980s, Johnson said that he had wanted to appear in the original Star Trek during its run on NBC from 1966 to 1969, but he was never cast.

In the late 1980s horror TV series Monsters, Johnson played an elder scientist in the episode Sleeping Dragon, in which he tries communicating with an ancient humanoid dinosaur that was awakened after a 65 million years-long slumber.

He shared an amusing anecdote:[citation needed] I was at a speaking engagement for MIT ... and I said ... the Professor has all sorts of degrees, including one from this very institution!

And that's why I can make a radio out of a coconut, and not fix a hole in a boat!Johnson once participated in the Ig Nobel award presentation ceremony, credited as "The Professor Emeritus of Gilligan's Island".