In the Israeli War of Independence he served in Palmach. He began his political career as a member of the movement Hashomer Hatzair, in which he filled a leadership role. He was one of the editors of their official newspaper Al Ha-Homa from 1939 to 1941. From 1944 to 1946 he was a member of kibbutz Mishmar HaEmek. He was founder and editor of the Israel Defense Forces official newspaper Bamahane ("In the Camp") from 1947 to 1950. During the 1950s he was a member of the editorial board of the newspaper Maariv and the editor of its literature section.
In his 1947 novel He Walked Through the Fields, which became the first play performed in the established State of Israel, the hero is a native-born Israeli, a "Sabra". The book won the Ussishkin Prize. It was adapted as a movie directed by Yosef Milo, who also directed its theatrical debut. In 1947 he founded the Israel Defense Forces newspaper Bamachane. He edited it until he was dismissed at the request of David Ben-Gurion for publishing an article about a celebration of the disbanding of Palmach. Thereafter he continually aroused scandals, more than any other Hebrew author of our time.
A survey made in the 50s by the Szold Institute found that two thirds of respondents preferred Shamir to all other Israeli authors. He was the most prominent representative of his generation of writers, much as Etgar Keret was to the writers of the 1990s.
From the first, Shamir was simultaneously an author, thinker, ideologue, and politician. Furthermore he was highly controversial, so that in his first decades he was nearly estranged from the very camp to which he originally belonged.
The hero of his book With His Own Hands: Alik's Story (1951) is his brother Alik who fell in the War of Independence. The book became an icon of that war. Alik's Story was translated into English, adapted into radio plays, and even merited an adaptation for television. It is one of the greatest Israeli bestsellers of all time, selling to date over 150,000 copies. It became part of the program of study in schools.
Two of Shamir's books, Under the Sun (1950) and That You Are Naked (1959) are autobiographical pieces based on his life in the thirties and forties. Shamir valued them far above his more famous works, considering them his best, although very few agree with him. Shamir wrote additional books about the members of his family: With His Own Heart about his father, and Not Far From the Tree about his family history.
Besides The King of Flesh and Blood, his most translated book was actually a lesser known children's book, The Fifth Wheel (1961). It is about the adventures of a kibbutznik, dispatched to bring a tractor from the port, who at every step meets various and sundry obstacles and adventures.
As a young writer Shamir became accustomed to the heavy hand of criticism. He especially took criticism from Joseph Klausner concerning The King of Flesh and Blood, whose central character is the Hasmonean king Alexander Jannæus. Menachem Begin recalled Klausner's words in a later day when Moshe Shamir, as a member of the Knesset, crossed the political lines from left to right to oppose the Israel-Egypt Peace Treaty. As the prime minister at the time, Begin spoke out against Shamir in the Knesset, indicating that Shamir's objections showed a lack of awareness of the historic moves taking place. He said to Shamir (in Hebrew):
Moshe Shamir also wrote poetry. However, most of his trade was in prose. He was a prolific author, publishing in the course of his life more than 25 books. Thus he is best recognized as a novelist and a playwright.
He died in Rishon LeZion at the age of 83. He was survived by his wife Tzvia.
His shift from left to right took a toll on him as the main literary societies, taking a dim view, banned him from membership.