An optician's son, Henri Nallet began his higher learning at the Institut d'études politiques de Bordeaux, from which he graduated first in his class in 1961.

In 1962, he earned a licentiate in public law, and then a graduate diploma (Fr: diplôme d’études supérieures) in political science in 1966, with a research paper on "Group Agriculture".

Active in youth movements, Nallet worked regularly with the Jeunesse Étudiante Chrétienne (JEC; sometimes "Young Christian Students" in English) beginning in 1961, before being elected its secretary-general in 1963.

[15] Nallet's interest in the agricultural world and his experiences in the JEC led him, in 1965, to become an instructor at the Rural Manager Training Institute (Fr: Institut de formation des cadres paysans; IFOCAP}, an association whose goal is to afford farmers holding responsibilities in professional organizations a higher level of training.

That same year, he was called by the heads of the Fédération nationale des syndicats d'exploitants agricoles (FNSEA; National Federation of Agricultural Holders' Unions), notably Michel Debatisse, the secretary-general, to become the mission manager for economic affairs.

His research activities gave rise to the publication of various studies, notably on trade unionism, agricultural politics, rural people's legal status, livestock breeding and dairy production.

Quickly thereafter he was called by the President of the Republic to the Élysée Palace, where he became a technical adviser, by decree on 6 July 1981, in charge of agricultural matters, communal problems and fishing.

During this period, he followed on the president's behalf all agriculture-related files, took care of relations with professional organizations and participated, with the then Minister of Foreign Affairs Roland Dumas, in negotiations to expand the EEC by admitting Spain and Portugal.

In early April 1985, François Mitterrand chose Nallet as his Minister of Agriculture[18] to succeed Michel Rocard, who had just resigned, in the portfolio.

The press at the time made much of this surprising nomination: Henri Nallet, then unknown to the French people, seeming much more like a "technician with a gift of political sense" than a "politician", as Le Monde's 6 April 1985 edition put it (he would not officially join the Socialist Party until 1987).

Despite the change of government, the former minister kept until June 1987 his functions as the World Food Council's (WFC) president, and his interest in the great debates in agricultural politics did not wane.

On 2 October, when there was a cabinet shuffle, he in fact became the Keeper of the Seals, that is, the Minister of Justice,[20] a portfolio that he kept after May 1991 in Édith Cresson's government, and until her resignation.

Nallet, the Keeper of the Seals, denounced the judge's action as "une équipée sauvage" (roughly "a wild escapade") and had him removed.

Beginning in 2008, he was part of the administrative council of the Emergency Rights Association, a humanitarian organization created in 1995, which fosters access to the law by those most deprived, and which participates in the struggle against exclusion.