The intended heir of his father's successful turbine business, Etnier entered Yale College, New Haven, Connecticut, in 1926, but soon transferred to Yale's art school. He bolted for South America within his first year, however. On his return after six months, he returned to Yale, but was soon invited to leave because of his poor academic record. His family next placed him at Haverford College, near Philadelphia, which led to a meeting with N. C. Wyeth, and on Wyeth's advice, a change--with parental blessing--to study at the Pennsylvania Academy of the Fine Arts in Philadelphia. He did not find true enthusiasm for his artistic pursuit, however, until he had the opportunity to work with Rockwell Kent at his studio in Ausable Forks, New York. His training was continued, and much influenced by summers spent in Woodstock, New York, in study with John Carroll, who also became a close friend. It was on sailing trips with Carroll that he was introduced to the Maine coast, which became the subject with which Etnier was most identified.
His first solo show was at Dudensing Brothers gallery, New York, in 1931; from 1932 into the 1960s he was represented in New York by Milch Galleries, and thereafter by the Midtown Gallery.
Etnier found his subject matter in his hometown of York, Pennsylvania, before forming his permanent attachment to Maine in the 1930s, when he bought an island at the mouth of the Kennebec River and designed and built on it his studio and home. Late in the decade he participated in the federal arts projects, executing a mural for the Spring Valley, New York, post office. During the second World War Etnier was on active service in the United States Navy.
In addition to his landscapes, and figure studies within the Maine landscape, Etnier's subjects were drawn from his frequent visits to the Caribbean--the southern coastal towns and islands reached by his favorite preoccupation, sailing.
In Academy annual exhibitions his works were awarded the Saltus Medal, 1955; an Altman prize, 1956; and Morse Gold Medal, 1964.