In January 1952, he joined the clandestine transitional leadership of Neo Destour, alongside notable figures including Farhat Hached and Hédi Nouira—tasked with resistance activities against the French occupation.

In August 1954, Mestiri became chief of staff to Mongi Slim, the minister of state from Neo Destour tasked with leading negotiations with France, culminating in Tunisia's internal autonomy a year later.

By September 1955, following Slim's appointment as Minister of the Interior in Tahar Ben Ammar's government, Mestiri played a key role in the negotiations that led to France recognizing Tunisia's independence on March 20, 1956.

In the first government formed by Habib Bourguiba on April 15, he was appointed Minister of Justice, where he was instrumental in the Tunisification of the judicial system, drafting new laws, and developing the Code of Personal Status.

During the final months of 1967, the agricultural and commercial reforms initiated by Ahmed Ben Salah, backed by President Bourguiba and the Neo-Destour party—later known as the Destourian Socialist Party (PSD)—met with significant opposition within Tunisia, despite efforts by official statements and the press to conceal this.

Mestiri voiced his opposition to the government's policies, highlighting public grievances against regional officials, questioning the state's role in managing economic enterprises, and criticizing the overzealous approach to the reforms.

On September 10, 1969, Ahmed Mestiri ended a twenty-month period of silence by expressing his support for President Bourguiba's decision to halt reforms and reassign portfolios from Ben Salah, retaining only the national education for him.

In a statement on October 4, Mestiri criticized the government's past actions and proposed a recovery program, advocating for the postponement of upcoming presidential and legislative elections due to Bourguiba's ill health.

Despite the MDS's initial success in the 1981 pluralist elections, the regime manipulated the results to ensure a PSD victory, an action widely criticized by both domestic and international observers for its blatant fraud.