Stewart joins Sotheby's as the chief executive officer effective immediately, succeeding Tad Smith, who has been with the company since 2015.
Stewart was the chief financial officer at Altice USA most recently and his other stints include serving as an investment banker at Morgan Stanley.
In June, Franco-Israeli cable magnate Drahi made a surprise move into the art world by snapping up Sotheby's in a $3.7 billion (2.9 billion pounds)deal, partly funded by the sale of some of his shares in Altice USA.
Altice USA, in a separate statement, named Michael Grau as its chief financial officer.
(Reporting by Soundarya J in Bengaluru; Editing by Subhranshu Sahu)