Naismith appeared in films such as Diamonds Are Forever (1971), Richard III (1955), Jason and the Argonauts (1963), Sink the Bismarck! (1960), Carrington VC (1954) and as Captain Edward Smith of the RMS Titanic in A Night to Remember (1958). He appeared on Broadway in the musical Here's Love in 1963 and played the non-singing role of Merlin in the 1967 film version of the musical Camelot. He was Judge Fulton in the TV series The Persuaders! (1971), with Tony Curtis and Roger Moore. He also starred in a children's ghost film The Amazing Mr Blunden (1972). He portrayed Emperor of Austria Franz Joseph in the BBC production Fall of Eagles (1974). Naismith played the Prince of Verona in the BBC Television Shakespeare version of Romeo & Juliet.

Outside of acting he was the landlord of the Rowbarge pub at Woolhampton, Berkshire and a keen cricket fan.

Naismith married, in 1939, Vera Bocca of Horden, County Durham.