Sellier remained an active socialist and collaborated with Jean Jaurès and Albert Thomas.

He voted with the majority to join the Communist Third International at the Tours Congress in November 1920, but in October 1921 was expelled from the French Communist Party and joined the Socialist French Section of the Workers' International (SFIO).

He remained a member of the SFIO for the rest of his life.

In 1909 Sellier was elected to the Puteaux municipal council in the western suburbs of Paris.

In 1910 he was elected to represent the canton of Puteaux in the general council of the Seine department, and resigned from the Ministry.

He was reelected to the general council in 1912 and 1919.

He was elected mayor of Suresnes in 1919, and held office until 1940.

His electoral program highlighted improvements to health, housing and urban renewal.

In 1925 the canton of Puteaux was divided, and Sellier was elected to represent the 2nd district, Suresnes and Nanterre-Sud, in the general council of the Seine.

He was reelected in 1929 and 1935.

He was general rapporteur of the budget of the Seine department from 1917 to 1920, and president of the general council in 1927–28.

Sellier ran unsuccessfully for election to the senate in January 1927.

He was elected senator for the Seine in the first round in the elections of 20 October 1935 on the Popular Front list.

He was Minister of Health in the first cabinet of Léon Blum from 4 June 1936 to 21 June 1937.
Sellier remained an active socialist and collaborated with Jean Jaurès and Albert Thomas.
He voted with the majority to join the Communist Third International at the Tours Congress in November 1920, but in October 1921 was expelled from the French Communist Party and joined the Socialist French Section of the Workers' International (SFIO).
He remained a member of the SFIO for the rest of his life.
In 1909 Sellier was elected to the Puteaux municipal council in the western suburbs of Paris.
In 1910 he was elected to represent the canton of Puteaux in the general council of the Seine department, and resigned from the Ministry. 
He was reelected to the general council in 1912 and 1919.
He was elected mayor of Suresnes in 1919, and held office until 1940.
His electoral program highlighted improvements to health, housing and urban renewal.
In 1925 the canton of Puteaux was divided, and Sellier was elected to represent the 2nd district, Suresnes and Nanterre-Sud, in the general council of the Seine.
He was reelected in 1929 and 1935.
He was general rapporteur of the budget of the Seine department from 1917 to 1920, and president of the general council in 1927–28.
Sellier ran unsuccessfully for election to the senate in January 1927.
He was elected senator for the Seine in the first round in the elections of 20 October 1935 on the Popular Front list.
He was Minister of Health in the first cabinet of Léon Blum from 4 June 1936 to 21 June 1937.
Henri Sellier