He studied at the Feagin School of Dramatic Art before joining Maurice Evans' theatrical company, with whom he made his Broadway debut with a walk-on in Henry IV, Part 1 (1939).

Nicol next appeared in Forward the Heart, and then as part of the original cast of Rodgers and Hammerstein's musical South Pacific (1949), playing one of the marines, but after a few weeks in the show he successfully auditioned to replace Ralph Meeker as Mannion in Mister Roberts, and was also made understudy to the play's star Henry Fonda.

Nicol was given a contract by Universal, and Sherman also directed his second film, Tomahawk (1951), in which he played a cavalry officer with a hatred of Indians.

Small roles as a prisoner of war in Target Unknown (1951) and a trainee pilot in Air Cadet (1951) preceded Nicol's first major part, co-starring with Frank Sinatra and Shelley Winters in the musical drama Meet Danny Wilson (1952).

Nicol returned to Universal (at a much larger salary than he had been getting as a contract player) to appear in two George Sherman films, The Lone Hand (1953) and Dawn at Socorro (1954).

Nicol then made three films in England, including the lead role in Face the Music (1954), and Ken Hughes' The House Across the Lake (1954).

Those British pictures kept me working, but they were really fast.Anthony Mann directed Nicol in his role as a navigator in Strategic Air Command (1955), and it was Mann who then gave the actor his best-remembered role as the weak psychopathic son of a patriarch rancher (Donald Crisp) who menaced Jimmy Stewart in The Man from Laramie (1955).

After a supporting role in Jacques Tourneur's Great Day in the Morning (1956) Nicol believed his Hollywood career was not progressing.

In 1956 he returned to Broadway to replace Ben Gazzara in the lead role of Brick, in Cat on a Hot Tin Roof.

We shot the picture in six weeks and it did very well, so I was happy with that.Nicol traveled to Italy when director Martin Ritt gave him a role in Five Branded Women (1959).

It was one of the happiest times of my life.One of his last assignments in Italy was another directorial credit, Then There Were Three, also known as Three Came Back, a World War II combat and spy actioner, which he also produced and was one of the co-stars, along with Frank Latimore.

Returning to the United States in 1961, he played Paul Anka's father in the thriller Look in Any Window (1961), with subsequent acting roles including The Twilight Zone episode "Young Man's Fancy" in 1962; two westerns, The Savage Guns (1962) and Gunfighters of Casa Grande (1964); Brandy (1964), Roger Corman's Bloody Mama (1969), based on the life of Ma Barker, and the independently mede religious horror The Night God Screamed.