robber baron

(chiefly US, idiomatic, usually derogatory) Especially in the 19th and early 20th centuries, a business tycoon who had great wealth and influence but whose methods were morally questionable and often unethical.

Example: 1900, Jack London, "The Man with the Gash,"
  Men who made it a custom to travel the trail to Dawson, likened him to a robber baron, perched in his fortress and exacting toll from the caravans that used his ill-kept roads.
1886, "A Robber Baron Muses," New York Times, 14 Mar., page 3:
  Still sails the Robber Baron's yacht in sunny Southern seas. Daily she jams her nose ashore, and daily takes on and puts off a fresh cargo of telegraph dispatches; and he who is idling for his liver's sake knows every night the tale of Wall-street's ticker and baiteth still without cessation his everlasting mouse trap.
1975, Michael Argyle, Bodily Communication, ISBN 9780823605507, page 206:
  An early operator in the field, Ivy Lee, is reported to have changed the image of John D. Rockefeller from robber baron to philanthropic old gentleman who loved to play golf and hand out shiny coins to children.