With the United States’ entry into World War II, he enlisted in the Army Intelligence Corps in 1942. Two years later he was assigned to a bomber group in Brindisi, Italy, ultimately becoming acting chief intelligence officer. In 1945 he returned home with a Bronze Star and the rank of captain. Discharged in 1945, he returned to Morningside Heights and Columbia but almost immediately decided to leave New York, and in the fall of 1946, he became assistant professor of philosophy and assistant to the dean of arts and sciences at Rutgers University. The following year he was promoted to assistant dean and associate professor.
In 1949, Mason Gross assumed the new position of provost, created by the trustees when president Robert Clothier became ill. As provost, he became the chief academic officer, and at the same time, attained the rank of full professor. Although he was the choice of many faculty members and administrators to succeed President Clothier in 1951, the trustees selected Lewis Webster Jones, a former university president. During the next eight years he continued to serve as provost, teach philosophy, and appear as “answer man” on two popular television quiz shows, Think Fast and Two for the Money, where he gained national acclaim for his scholarship and genial wit. In 1958 he took on the additional title of vice president and with the abrupt resignation of President Jones, he became acting president. In February 1959 Mason Gross was unanimously selected by the Board of Governors as the 16th president of Rutgers.
It was during the turbulent decade of the 1960s under the leadership of Mason Gross when Rutgers witnessed unprecedented growth and development. In 1959 the first of three bond issues was passed by the citizens of New Jersey, enabling the university to embark upon a $75 million building program. By 1964 enrollment had doubled with more than 12,000 full-time undergraduate students. A second public referendum yielded approximately $19 million for Rutgers, and President Gross continued to campaign for funds for the university. An additional $68 million was secured in 1968. As a result of increased public support, construction took place on every campus of the University. Seven buildings, including a library, a student center, and a law school building were erected on the 18.3-acre complex in Newark. Large-scale development occurred on the 16-acre campus in Camden, including a law school complex. A new library and dining hall were constructed at Douglass College, and new dormitories and classroom facilities sprung up on the Rutgers College campus. In 1964 Rutgers acquired from the federal government 540 acres of the former Camp Kilmer army base and the first buildings were erected on the Kilmer-area campus, where Livingston College opened in 1969. Scientific research and teaching facilities emerged on the Busch Campus.