An American-born Roman Catholic priest of Hungarian descent, serving in the Diocese of Hartford and later in the Diocese of Bridgeport. He was ordained a priest on May 25, 1929. At the beginning of his priestly ministry, he served at St. Patrick’s Parish in New Haven, and in 1933 he was transferred to St. László Hungarian Parish in South Norwalk. He served as the parish priest for more than four decades, until his retirement around 1977–1978. During his long tenure, he became a defining figure for the parish’s second-generation community of Hungarian descent, which was gradually becoming English-speaking.
His relationship with Hungarian identity, however, was not entirely without issues: In the context of József Mindszenty’s 1974 trip to the United States, a study notes that Lengen—although he knew Hungarian—tended to use English as the liturgical language, for which he was criticized by Mindszenty.

Pope John XXIII conferred upon him the title of papal prelate, and from then on he was addressed as Monsignor.
He served as vice president of the American Hungarian Catholic League. After 1956, he regularly sent financial support and packages worth 10–20 dollars to Hungary under the pseudonym János Szabó. He retired in 1977.
In 1989, he was honored at a solemn Mass marking the sixtieth anniversary of his ordination to the priesthood and his ninetieth birthday. He passed away on June 9, 1999; his grave is located at St. Michael’s Catholic Cemetery in Stratford.