Since its launch in June 1998, the service has carried over 1.9 million customers on more than 13,500 flights.
Shannon Airport's managing director, Andrew Murphy, called the anniversary an "important milestone" in the Irish gateway's history.
He enthused: "Not only does it mark 20 years of flying, it marks 20 years of supporting tourism and industry in this region.
"Newark is such an accessible airport and New York is, of course, the most popular city in terms of transatlantic services between Ireland and the US.
"We appreciate United Airlines’ commitment in serving Shannon for the last 20 years and look forward to celebrating the next milestone.”
The airline's managing director sales for the United Kingdom and Ireland, Bob Schumacher, said: “We are delighted to mark 20 years of our non-stop service from Shannon to New York/Newark.
“Since 1998, our service has allowed our customers from the West of Ireland to take advantage of our extensive route network from our New York/Newark hub that today offers 75 non-stop connections across the US, Canada and Mexico.”